SyncChannel<T> can move 'T: !Send' to other threads

Affected versions of this crate unconditionally implement Send/Sync for SyncChannel<T>. SyncChannel<T> doesn’t provide access to &T but merely serves as a channel that consumes and returns owned T. Users can create UB in safe Rust by sending T: !Send to other threads with SyncChannel::send/recv APIs. Using T = Arc<Cell<_> allows to create data races (which can lead to memory corruption), and using T = MutexGuard<T> allows to unlock a mutex from a thread that didn’t lock the mutex.