Support an exit_event_loop() function

[guybedford]

This works similarly to something like an exit() call where it implies that the process is done and that we no longer need to spin on async tasks.

This seems to make sense to me in that promise interest is about determining event loop completion, which may result in stalls before the response, while exiting accepts that any previously in-progress tasks may be abandoned, no matter how much interest they previously had.

Would be great to discuss further.