Add a futures type that can unify our future functionality

Move the currently existing green future executor and associated
code to a new futures types module so that it can be accessed from
this new location (TODO: deprecate the old location and link the
old to the new for one release so that we can remove the old link
in N + 1 release).

This unifies the API that the existing pool (thread or process) future
executors and the green thread pool future executor, and the newly added
synchronous executor (replacing the previous `make_completed_future`
function) provide so there usage is as seamless as possible.

Part of blueprint top-level-types

Change-Id: Ie5500eaa7f4425edb604b2dd13a15f82909a673b