Channel implementation with zero capacity

[johnhungerford]

Streaming from channels currently uses drainUpTo on the underlying Queue to grab chunks from the channel. Since there is no queue for zero-capacity channels, they will necessarily stream one element at a time (that is, chunks of size 1 -- essentially a loop over take), even though there may be multiple pending puts that could be drained.

It probably makes sense to provide a special implementation for channels of capacity zero that has no main queue and simply passes values directly from the pending puts to the pending takes. drain and drainUpTo could be specialized to drain the puts. Since there is no queue for these to pass to, there is no issue with ordering to worry about (which is why regular channels only drain the main queue and not the puts).