Made isEmpty and isFull useful on unbuffered channels.
Made isEmpty and isFull useful on unbuffered channels.

If no threads are waiting to send to the channel, it is empty.
If no threads are waiting to receive from the channel, it is full.

Both can be true (if no threads at all are waiting), but they cannot both be false at the same time.

glessard/swift-channels,glessard/swift-channels