Support up to 16 seeds without heap allocation

[snawaz]

#106 increased the limit from 4 to 8 seeds, but Solana runtime supports up to 16 seeds. Therefore, we should support up to 16 seeds as well.

Consider writing a macro to avoid verbosity and to make it a reusable utility:

let seeds_to_validate: &[&u8]] = seeds_as_slices!(args.seeds);

Consider zero-copy as well. Currently, DelegateArgs allocates heap-memory for Vec.