at_coroutine_exit can probably break scheduler affinity

[ispeters]

I can't know without writing a unit test, but I'm pretty certain this breaks Scheduler affinity:

template <typename Scheduler>
task<void> foo(Scheduler someOtherScheduler) {
  co_await at_coroutine_exit([=]() noexcept {
    return schedule(someOtherScheduler);
  });
}

There's no infrastructure in the implementation of at_coroutine_exit to prevent the above from breaking the affinity invariant and no tests confirming that it doesn't break the invariant so it's almost certainly broken.

Note: the magic behaviour of co_await schedule(s); in a coroutine maintains the affinity invariant by using at_coroutine_exit to reschedule onto the correct Scheduler so fixing this will require some careful thought.