More generic From<[T; N]> and Into<[T; N]> implementations

[Qqwy]

Currently, these implementations are hard-coded for certain array lengths.

This was necessary on old Rust versions in which

• const generics were not possible, and
• array traits were restricted to max lengths of 32.
  But since v1.47 this is no longer the case.

By enabling the consts feature on typenum, we get access to the U<const N: usize> type. And this is what lets us write a single implementation for From and for Into that will work for all lengths!

[Qqwy]

Currently I am working around this in my own code by transmuting directly (as the current hard-coded implementations of From/Into do as well) which works, but is hacky as it makes my code deeply dependent on the internals of this crate.

[novacrazy]

This is now implemented in the 1.0 branch. However, to ease use with const-generics in user libraries, I'm considering something like:

pub unsafe trait IntoArrayLength {
    type ArrayLength: ArrayLength;
}

unsafe impl<const N: usize> IntoArrayLength for Const<N>
where
    Const<N>: ToUInt,
    U<N>: ArrayLength,
{
    type ArrayLength = U<N>;
}

pub type ConstArrayLength<const N: usize> = <Const<N> as IntoArrayLength>::ArrayLength;

impl<T, const N: usize> From<[T; N]> for GenericArray<T, ConstArrayLength<N>>
where
    Const<N>: IntoArrayLength,

Cuts down on one where bound.