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

dlight:

I think the right feature for abbreviating long derive lines is some kind of derive alias.

I agree that this is a useful feature, but I think having Eq derive PartialEq as well and Copy derive Clone as well is good even in presence of that feature.

That is, if we were going to design a Rust-like language today, I think having #[derive(Eq)] is clearly a better choice than #[derive(PartialEq, Eq)]. But the current state of Rust and whether it makes sense to use hacks/editions to prevent breakage is another debate.