adding aligner to BigInt

For our project we need the BigInt type to have a certain alignment. Would you consider modifying the struct definition to be something like
```rust
#[derive(Copy, Clone, PartialEq, Eq, Hash)]
#[must_use]
#[repr(C)]
pub struct BigInt<A, const N: usize>{
    // BigInt is aligned to (at least) A
    _aligner: [A; 0],
    data: [u64; N]
}
```
If yes, I can make the changes and open a PR