It takes a lot of boilerplate to create a task and establish a way to talk to
it. This function simplifies that, allowing you to write something like
'worker(f).chan <| start'. Implementation is very unsafe and only works for a
few types of channels, but something like this is very useful.
Updated the MapReduce protocol so that it's correct more often. It's
still not perfect, but the bugs repro less often now.
Also found a race condition in channel sending. The problem is that
send and receive both need to refer to the _unread field in
circular_buffer. For now I just grabbed the port lock to send. We can
probably get around this by using atomics instead.
