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

Summary

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.