Make easier to add shuffle to other types

[leighmcculloch]

What

Change how shuffle is implemented and provided so that the functionality is provided by implementors of a trait.

Also, change how it works so that it mutates the existing value. But provide a to_shuffled function as well.

Why

There's a few reasons for the different changes that were difficult to break apart which is why I've submitted them all in this one PR.

Providing the shuffle functionality via a trait allows us to define shuffle behavior on a wider variety of types, other than only types that implement IntoVal<_, Vec<Val>>. For example, we'd be able to provide the functionality on [T; N] as well or some other types in the future.

Providing the shuffle functionality as a mutation of the type also provides some optimizations in cases where the developers wants to shuffle something without allocating a new copy of it on the stack or elsewhere. Even though the functionality is changed to mutate, it's still possible for the Vec case for us to include a to_shuffled function that provides a copy. In the end this communicate clearer.

I implemented ToShuffled for all Shuffle, but maybe that's a mistake. Blanket implementations often come back to bite us.

Note

This is a breaking change. There isn't really a good way to deprecate this and reuse the fn names. We're still in pre-release though, so I think that's okay.