Enable typenum's const-generics feature

[tarcieri]

Though this doesn't immediately leverage the feature, it could potentially be useful in implementing e.g. ArrayN or as a replacement for AssociatedArraySize.

Enabling the feature does make it accessible for downstream users as hybrid_array::typenum.

[tarcieri]

I tried using typenum::U<N> in various ways and all of them were somewhat unsatisfying.

The main problem is that it's built on ToUInt impls on Const<N>, so those need to be propagated in bounds.

AssociatedArraySize (despite the unwieldy name) makes for more convenient bounds, namely bounding on [T; N]: AssociatedArraySize + AsRef<Array<T, U>> versus:

where
    Const<N>: ToUInt,
    [T; N]: AsRef<Array<T, U>>