`bevy_ptr`'s `ConstNonNull` does not implement `Copy` or `Clone`

## What problem does this solve or what need does it fill? / What solution would you like?

[Parity with `NonNull`](https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#trait-implementations).

## What alternative(s) have you considered?

Not doing the above.