This is a draft of the `tractor` way to implement the example from the
"processs pool" in the stdlib's `concurrent.futures` docs:

https://docs.python.org/3/library/concurrent.futures.html#processpoolexecutor-example

Our runtime is of course slower to startup but once up we of course get
the same performance, this confirms that we need to focus some effort
not on warm up and teardown times.  The mp forkserver method definitely
improves startup delay; rolling our own will likely be a good hot spot
to play with.

What's really nice is our implementation is done in approx 10th the code ;)

Also, do we want offer and interface that yields results as they arrive?

Relates to #175