Support up to 16 seeds without heap allocation

#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:

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

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