Implement `Fill` for derived array types

[dhardy]

E.g. it would be good to support types like [[u64; 32]; 4] or [[u64]].

[dhardy]

With specialization, we can implement our special filling routines via the Standard distribution, then make our Fill trait (#940) a wrapper or remove it completely.

Until then, and especially without const_generics, there are too many permutations of types to support.