Added

• Added the ability to get global and group slot numbers for each future. Slot numbers are unique for as long as the future lives and are freed once the future is complete. Slot numbers are also compact because they start from 0, and the smallest possible number is assigned to them.

Changed

• As a result of the ability to get slot numbers, the stream must now return a closure of FnOnce(FutureQueueContext) -> impl Future.