Add Read from mutable slices

[kaidokert]

Adding a Read implementation for mutable slice, which is useful in some contexts.

[npmccallum]

@kaidokert I support this in theory. Please make sure the tests pass.

[kaidokert]

Oof, I was mistaken, this doesn't actually work for std, it's fine for no_std which i tested.

&mut [u8] doesn't implement std::io::Read, and thus the blanket Read implementation can't be used due to orphan rule. I don't know if there's a workaround at all for this, and diverging from std::io::Read isn't a great idea in the first place.

Converted to a draft, but i don't think there's a practical way to implement this and no great benefit.