Make unsound opt-in feature

[JackThomson2]

Description

Updating of the PR: #51
Rebased against current main and re-ran the performance tests to compare the difference.

Unsure if the unsound should be opt-in or opt-out, but left match previous PR for now

Tested on c7g (Graviton 3)

 name                                                    safe ns/iter   unsound ns/iter  diff ns/iter   diff %  speedup 
 benches::bench_bincode_deserialize                      182,927        178,688                -4,239   -2.32%   x 1.02 
 benches::bench_bincode_serialize                        24,170         24,311                    141    0.58%   x 0.99 
 benches::bench_bitcode_decode                           131,508        131,705                   197    0.15%   x 1.00 
 benches::bench_bitcode_encode                           16,323         14,305                 -2,018  -12.36%   x 1.14 
 benches_borrowed::bench_bincode_deserialize             91,928         91,571                   -357   -0.39%   x 1.00 
 benches_borrowed::bench_bincode_serialize               23,047         23,035                    -12   -0.05%   x 1.00 
 benches_borrowed::bench_bitcode_decode                  12,158         12,179                     21    0.17%   x 1.00 
 benches_borrowed::bench_bitcode_encode                  16,477         14,599                 -1,878  -11.40%   x 1.13 
 bool::test2::bench_bool_vecs_decode                     4,988          4,968                     -20   -0.40%   x 1.00 
 bool::test2::bench_bool_vecs_encode                     365            226                      -139  -38.08%   x 1.62 
 bool::test::bench_bool_vec_decode                       92             91                         -1   -1.09%   x 1.01 
 bool::test::bench_bool_vec_encode                       61             61                          0    0.00%   x 1.00 
 buffer::tests::bench_registry100_get                    19             19                          0    0.00%   x 1.00 
 buffer::tests::bench_registry10_get                     14             14                          0    0.00%   x 1.00 
 buffer::tests::bench_registry1_get                      1              1                           0    0.00%   x 1.00 
 derive::array::tests::bench_u8_array_vecs_decode        4,480          4,512                      32    0.71%   x 0.99 
 derive::array::tests::bench_u8_array_vecs_encode        431            436                         5    1.16%   x 0.99 
 derive::duration::tests::bench_duration_vec_decode      833            837                         4    0.48%   x 1.00 
 derive::duration::tests::bench_duration_vec_encode      1,094          1,099                       5    0.46%   x 1.00 
 derive::empty::tests::bench_phantom_data_vec_decode     8              8                           0    0.00%   x 1.00 
 derive::empty::tests::bench_phantom_data_vec_encode     12             12                          0    0.00%   x 1.00 
 derive::impls::tests::bench_tuple_vec_decode            10,250         10,248                     -2   -0.02%   x 1.00 
 derive::impls::tests::bench_tuple_vec_encode            8,975          8,976                       1    0.01%   x 1.00 
 derive::map::test::bench_btree_map_decode               2,009          2,012                       3    0.15%   x 1.00 
 derive::map::test::bench_btree_map_encode               661            662                         1    0.15%   x 1.00 
 derive::map::test::bench_hash_map_decode                3,434          3,486                      52    1.51%   x 0.99 
 derive::map::test::bench_hash_map_encode                655            660                         5    0.76%   x 0.99 
 derive::option::tests2::bench_option_u16_vec_decode     769            762                        -7   -0.91%   x 1.01 
 derive::option::tests2::bench_option_u16_vec_encode     703            703                         0    0.00%   x 1.00 
 derive::option::tests::bench_option_vec_decode          7,257          7,261                       4    0.06%   x 1.00 
 derive::option::tests::bench_option_vec_encode          6,349          6,344                      -5   -0.08%   x 1.00 
 derive::result::tests::bench_result_vec_decode          907            912                         5    0.55%   x 0.99 
 derive::result::tests::bench_result_vec_encode          2,193          2,229                      36    1.64%   x 0.98 
 derive::variant::tests::bench_bool_enum_vec_decode      105            102                        -3   -2.86%   x 1.03 
 derive::variant::tests::bench_bool_enum_vec_encode      89             89                          0    0.00%   x 1.00 
 derive::vec::test::bench_binary_heap_decode             208            209                         1    0.48%   x 1.00 
 derive::vec::test::bench_btree_set_decode               1,436          1,435                      -1   -0.07%   x 1.00 
 derive::vec::test::bench_btree_set_encode               368            370                         2    0.54%   x 0.99 
 derive::vec::test::bench_hash_set_decode                3,234          3,236                       2    0.06%   x 1.00 
 derive::vec::test::bench_hash_set_encode                310            310                         0    0.00%   x 1.00 
 derive::vec::test::bench_linked_list_decode             5,481          5,467                     -14   -0.26%   x 1.00 
 derive::vec::test::bench_linked_list_encode             458            458                         0    0.00%   x 1.00 
 derive::vec::test::bench_vec_decode                     41             41                          0    0.00%   x 1.00 
 derive::vec::test::bench_vec_deque_decode               41             41                          0    0.00%   x 1.00 
 derive::vec::test::bench_vec_deque_encode               43             43                          0    0.00%   x 1.00 
 derive::vec::test::bench_vec_encode                     41             41                          0    0.00%   x 1.00 
 f32::tests2::bench_f32_vecs_decode                      4,582          4,595                      13    0.28%   x 1.00 
 f32::tests2::bench_f32_vecs_encode                      650            681                        31    4.77%   x 0.95 
 f32::tests::bench_f32_vec_decode                        454,806        456,517                 1,711    0.38%   x 1.00 
 f32::tests::bench_f32_vec_encode                        715,469        715,775                   306    0.04%   x 1.00 
 fast::tests::bench_next_unchecked                       3,465          3,465                       0    0.00%   x 1.00 
 fast::tests::bench_next_unchecked_fast                  1,930          1,931                       1    0.05%   x 1.00 
 fast::tests::bench_push                                 2,445          2,455                      10    0.41%   x 1.00 
 fast::tests::bench_push_fast                            4,285          4,288                       3    0.07%   x 1.00 
 fast::tests::bench_push_unchecked                       2,813          2,864                      51    1.81%   x 0.98 
 fast::tests::bench_push_unchecked_fast                  3,728          3,721                      -7   -0.19%   x 1.00 
 fast::tests::bench_reserve                              691            692                         1    0.14%   x 1.00 
 fast::tests::bench_reserve_fast                         585            584                        -1   -0.17%   x 1.00 
 histogram::tests::bench_histogram_parallel_10           185            185                         0    0.00%   x 1.00 
 histogram::tests::bench_histogram_parallel_100          247            246                        -1   -0.40%   x 1.00 
 histogram::tests::bench_histogram_parallel_1000         847            846                        -1   -0.12%   x 1.00 
 histogram::tests::bench_histogram_parallel_10000        6,864          6,865                       1    0.01%   x 1.00 
 histogram::tests::bench_histogram_simple_10             54             54                          0    0.00%   x 1.00 
 histogram::tests::bench_histogram_simple_100            162            162                         0    0.00%   x 1.00 
 histogram::tests::bench_histogram_simple_1000           1,193          1,191                      -2   -0.17%   x 1.00 
 histogram::tests::bench_histogram_simple_10000          11,403         11,402                     -1   -0.01%   x 1.00 
 int::test2::bench_u16_vecs_decode                       3,962          3,964                       2    0.05%   x 1.00 
 int::test2::bench_u16_vecs_encode                       290            201                       -89  -30.69%   x 1.44 
 int::tests::bench_u16_vec_decode                        111            109                        -2   -1.80%   x 1.02 
 int::tests::bench_u16_vec_encode                        94             95                          1    1.06%   x 0.99 
 pack::tests::bench_pack_arithmetic16                    48             48                          0    0.00%   x 1.00 
 pack::tests::bench_pack_arithmetic2                     26             26                          0    0.00%   x 1.00 
 pack::tests::bench_pack_arithmetic3                     102            102                         0    0.00%   x 1.00 
 pack::tests::bench_pack_arithmetic4                     44             44                          0    0.00%   x 1.00 
 pack::tests::bench_pack_arithmetic6                     52             52                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_arithmetic16                  25             25                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_arithmetic2                   38             38                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_arithmetic3                   148            148                         0    0.00%   x 1.00 
 pack::tests::bench_unpack_arithmetic4                   41             41                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_arithmetic6                   90             90                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than16             7              7                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than16_histogram   8              8                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than2              7              7                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than256            3              3                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than256_histogram  19             19                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than2_histogram    8              8                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than3              7              7                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than3_histogram    12             12                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than4              7              7                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than4_histogram    8              8                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than6              7              7                           0    0.00%   x 1.00 
 pack::tests::bench_unpack_bytes_less_than6_histogram    11             11                          0    0.00%   x 1.00 
 pack::tests::bench_unpack_histogram16                   461            460                        -1   -0.22%   x 1.00 
 pack::tests::bench_unpack_histogram3                    791            782                        -9   -1.14%   x 1.01 
 pack::tests::bench_unpack_histogram4                    680            677                        -3   -0.44%   x 1.00 
 pack::tests::bench_unpack_histogram6                    571            569                        -2   -0.35%   x 1.00 
 pack_ints::tests::bench_pack_u008_max                   100            100                         0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u008_no_pack               35             35                          0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u008_random                74             74                          0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u008_zero                  69             69                          0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u016_max                   182            183                         1    0.55%   x 0.99 
 pack_ints::tests::bench_pack_u016_no_pack               59             59                          0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u016_random                67             68                          1    1.49%   x 0.99 
 pack_ints::tests::bench_pack_u016_zero                  154            154                         0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u032_max                   290            290                         0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u032_no_pack               112            112                         0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u032_random                121            122                         1    0.83%   x 0.99 
 pack_ints::tests::bench_pack_u032_zero                  215            215                         0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_u064_max                   506            510                         4    0.79%   x 0.99 
 pack_ints::tests::bench_pack_u064_no_pack               216            211                        -5   -2.31%   x 1.02 
 pack_ints::tests::bench_pack_u064_random                224            220                        -4   -1.79%   x 1.02 
 pack_ints::tests::bench_pack_u064_zero                  353            359                         6    1.70%   x 0.98 
 pack_ints::tests::bench_pack_u128_max                   2,155          2,153                      -2   -0.09%   x 1.00 
 pack_ints::tests::bench_pack_u128_no_pack               477            450                       -27   -5.66%   x 1.06 
 pack_ints::tests::bench_pack_u128_random                476            474                        -2   -0.42%   x 1.00 
 pack_ints::tests::bench_pack_u128_zero                  1,747          1,744                      -3   -0.17%   x 1.00 
 pack_ints::tests::bench_pack_usize_max                  506            511                         5    0.99%   x 0.99 
 pack_ints::tests::bench_pack_usize_no_pack              211            211                         0    0.00%   x 1.00 
 pack_ints::tests::bench_pack_usize_random               225            220                        -5   -2.22%   x 1.02 
 pack_ints::tests::bench_pack_usize_zero                 353            359                         6    1.70%   x 0.98 
 pack_ints::tests::bench_unpack_u008_max                 62             62                          0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u008_random              3              3                           0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u008_zero                44             43                         -1   -2.27%   x 1.02 
 pack_ints::tests::bench_unpack_u016_max                 111            111                         0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u016_random              4              4                           0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u016_zero                81             81                          0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u032_max                 190            189                        -1   -0.53%   x 1.01 
 pack_ints::tests::bench_unpack_u032_random              4              4                           0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u032_zero                106            105                        -1   -0.94%   x 1.01 
 pack_ints::tests::bench_unpack_u064_max                 313            313                         0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u064_random              4              4                           0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u064_zero                152            152                         0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u128_max                 1,185          1,184                      -1   -0.08%   x 1.00 
 pack_ints::tests::bench_unpack_u128_random              4              4                           0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_u128_zero                769            769                         0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_usize_max                313            313                         0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_usize_random             4              4                           0    0.00%   x 1.00 
 pack_ints::tests::bench_unpack_usize_zero               152            152                         0    0.00%   x 1.00 
 str::tests2::bench_str_vec_decode                       1,374,455      1,369,288              -5,167   -0.38%   x 1.00 
 str::tests2::bench_str_vec_encode                       96,575         57,505                -39,070  -40.46%   x 1.68 
 str::tests::bench_is_ascii                              405            405                         0    0.00%   x 1.00 
 str::tests::bench_is_ascii_simd                         421            416                        -5   -1.19%   x 1.01 
 str::tests::bench_str_tuple_decode                      59             59                          0    0.00%   x 1.00 
 str::tests::bench_str_tuple_encode                      92             92                          0    0.00%   x 1.00