FastQueue's Node with a Void pointer (and only one memory allocation per enqueue.)

If type Node<T> contains an UnsafeMutablePointer<Node<T>>, the runtime crashes during memory allocation.
Replacing the UnsafeMutablePointer with an UnsafeMutablePointer<Void> is a functional workaround.
The previous version was also a workaround, though a worse one. The speed difference isn't much, if anything.

rdar://19343981
Changed from (mis)using semaphore to OSSpinLock in the Swift-native queues.

It doesn't help the speed, though.
Cleanup

(with an update to Time and Interval)
chan-benchmark update to take advantage of the better Interval.
Eliminated memory allocations in one of the data transfer paths of 'QUnbufferedChan'

This makes a memory leak somewhat inconceivable, since the remaining memory allocation only occurs when a recipient is waiting for data.