Allocation with shared_arena

[lenianiva]

1. Add allocation traits in src/alloc.rs
2. Add benchmarks

I have tested so far that the allocation trait commit 8c663871d00e1fbf90947792e074dac68b59d23d is not slower than 37e29a352fb55187ac504b9df264fde9ca4af713. Currently I'm waiting for shared_arena to have a weak pointer before I can proceed.