Strongly typed bitvector-lengths

[djrenren]

Now that const generics are stable, we could have a pub struct BV<'ctx, const N: usize>.
This would prevent any mix ups when you have multiple sizes of bitvector floating around. I don't have time to put together a PR for this atm, but it would be really neat and I think not too difficult.

[GreenBeard]

If anyone decides to implement this please make sure that the ability to have bitvectors with sizes not known at compile time is not lost. I think this means that two versions of BV may be needed? I'm not entirely sure as I am not very familiar with Rust const generics.

[toolCHAINZ]

I think this could now be done cleanly without removing the flexibility of the current BV wrapper.

We could introduce SizedBV<const N: usize> and implement all the "binops" on it with only BVs of the same size. We could make BV::from_u64 and friends actually return SizedBV. We would implement From<SizedBV> for BV. Since all binops now take Into<BV> arguments, you could use a sized BV anywhere a normal one can be used. But if you are using only SizedBV, you get the static sized guarantees.

If you Add a BV to a SizedBV, what comes out the other side? The expression will fail to build at runtime if the size is not correct, so I think you could actually type this as returning SizedBV.

We could maybe also make this just a tuple struct around BV and maybe impl Deref to avoid duplicated API surface.