Removing `Key` bound from struct definition

I'm not seeing any real reason for (for example) [HopSlotMap](https://github.com/orlp/slotmap/blob/c905b6ced490551476cb7c37778eb8128bdea7ba/src/hop.rs#L148) to have a `Key` bound on the generic `K`. This is against the general Rust style guide as it leads to infectious `where` clauses, and prevents being generic over it in more permissive GATs without awkward workarounds.