Atom is just a wrapper around &str, so could be Copy.

I thought previously that we should avoid making Atom Copy in case we had a future reason for them not to be, like some form of reference-counting.

But now I can't see any such reason. Even if we store hash of string inside Atom (#46), that wouldn't prevent Atom being Copy.

This change would remove a ton of atom.clone() from all over the codebase.