OwnedKey::from_bytes should operate on things like &[u8; 4] when key size is fixed

[uint]

This sounds simple but would require digging into some type-fu.

It looks like the big "behavior" match thing would have to go to make this possible. Probably a good design choice anyway.

The benefit is implementing custom key types becomes a little easier for devs (no need for extra error handling when keys are fixed size).