CVE-2020-36445: Data races in convec

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

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