Cache<K>: Send/Sync impls needs trait bounds on `K`

Affected versions of this crate unconditionally implement Send/Sync for Cache<K>.
This allows users to insert K that is not Send or not Sync.

This allows users to create data races by using non-Send types like Arc<Cell<T>> or Rc<T> as K in Cache<K>. It is also possible to create data races by using types like Cell<T> or RefCell<T> (types that are Send but not Sync).
Such data races can lead to memory corruption.