Implement `ConstInstantiable` for arrays and slices

[AlexTMjugador]

It can be useful to have maps where values are slices or arrays. Given that Rust stabilized the basic const generics we need on Rust 1.51 and the project has a MSRV of 1.56, there is no reason I can see for not implementing this required trait for arrays and slices.

These changes implement such trait for the described types, as long as the contained type implements ConstInstantiable itself.