Make `#[derive(Copy)]` enough for both `Clone` and `Copy`

I assumed you want to implement this change before the nearest edition, and only make the hack not required on an edition, by virtue of always generating a Clone impl. To that I responded that it won't help the compiler complexity.

If you meant to wait until an edition and only then apply this change, then you're right - it won't require much complexity in the compiler, though we still need to consider the ecosystem churn.