Implement Copy and Clone for Handle<T> where T: !Copy

[gmorenz]

Derive only derives these impls for types where the generics satisfy them, but shared references (and thus Handle) can implement them for any type T.

I've run into this a few times while playing around now, because Cell doesn't implement Copy. It's not entirely clear that users of the library will run into this, but at worst it doesn't hurt to define these generally.

[gmorenz]

PS. It's also pretty likely that users will run into this if/when Handle is used more broadly. Right now the reason no one is running into this is because for the most part &/&mut referencess into RootedGuard are used.