Currently, There is two types of Executor in `concurrent.futures`.

They are `ThreadPoolExecutor` and `ProcessPoolExecutor`, there is GIL limitation of `ThreadPoolExecutor` and `ProcessPoolExecutor` need to use `pickle` which have limitation on decorated function.

It would be nice to add new Executor `UnixForkExecutor` which is based on `os.fork` and shared memory (to return future).

The drawback of UnixForkExecutor can only be used on Unix platform.

This is not a problem, because the majority people use python for parallel computing on Unix platform.

Anyone have any opinion about it?