Drop explicit support for multiprocessing #522
Description
The multiprocessing support:
- is complex and hard to test
- doesn't seem useful in practice, and as far as I'm aware isn't actually being used by any projects
- is unnecessarily specific - there are many ways to execute code in a remote process, and multiprocessing is just one of those
- is the main impediment to solving Rethink exception handling #318, since it's the reason that we need to serialize exceptions
We could instead document patterns for integration of Traits Futures with multiprocessing and other out-of-process remote execution.