Better leverage `typenum::Const<N>`?

[tarcieri]

When the const-generics feature of typenum is enabled, it provides the generic_const_mappings module which contains a Const<N> ZST with ToUInt impls automatically generated by a build script:

https://docs.rs/typenum/latest/typenum/generic_const_mappings/index.html

The typenum::U<N> type alias provides glue between the two types.

I investigated using this as e.g. an alternative to AssocArraySize in #39, but as noted in that issue it involves a viral bound:

impl <const N: usize> ...
where
    ...
    [T; N]: AsRef<ArrayN<T, N>>
    typenum::Const<N>: typenum::ToUInt

where the current approach allows for:

impl <const N: usize> ...
where
    ...
    [T; N]: AssocArraySize + AsRef<ArrayN<T, N>>

[tarcieri]

Something else I experimented with was trying to use typenum::U<$len> in the impl_array_size! macro so it only has to take numeric parameters rather than a 0 => U0, 1 => U1 mapping.

For whatever reason, that didn't work:

error[E0119]: conflicting implementations of trait `traits::ArraySize`
  --> src/sizes.rs:8:13
   |
8  |             unsafe impl ArraySize for typenum::U<$len> {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |             |
   |             first implementation here
   |             conflicting implementation
...
19 | impl_array_size!(0, 1);
   | ---------------------- in this macro invocation

[tarcieri]

Another reason to prefer the AssocArraySize approach is that it allows us to accept either ArrayN<T, N> or [T; N] easily/interchangeably after #43 lands:

pub fn foo<Arr, const N: usize>(arr: &Arr)
where
    Arr: AssocArraySize + AsRef<ArrayN<T, N>>
    AssocArraySize::Size: [typenum bounds here]
{
    let hybrid_array = arr.as_ref();
    ...
}

Also, we're ultimately constrained by which typenum::U# we impl ArraySize for, i.e. we don't support arbitrary N, so using typenum::Const<N>/U<N> doesn't accurately reflect that (in addition to having uglier/more complex bounds).

Given all that, I think I've exhausted all of the immediate potential usages of typenum::Const<N>, so I'm going to go ahead and close this. Hopefully if the subject comes up, it's at least a nice log of what possibilities were already researched.