Another speedup by taking over memory management. (And filename changes)

FastQueue<T> is nearly as fast as PointerQueue<T>, while being completely Swift-native. It will do worse under contention, but hopefully there won't be many cases of that.

Being Swift-native, this one will work on iOS (unlike the hybrid queues). It might help make an unbuffered channel usable there. To this end, Queue and FastQueue now incorporate semaphore protection.