Data races in cache

An issue was discovered in the cache crate through 2020-11-24 for Rust. 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.