Tidy `StopHandle` (#425)

* Experimental generic driver for methods

* Move FutureDriver to its own module

* "Infallible" sync methods + fmt

* Drive all methods to completion before background_task closes

* Testing a different stop handle

* Added the waker for the monitors

* Fix the test

* Fix a possible deadlock

* fmt

* Explain strong_count check in StopHandle Drop impl

* Clippy tweak

Co-authored-by: Niklas Adolfsson <[email protected]>

* Added comment on atomicbool ordering

Co-authored-by: Niklas Adolfsson <[email protected]>