Resolves #25.

The crash is caused by stack overflow. Coroutine contnuations can be nested, and the depth of nesting can exceed the available stack space. Such a nesting occurs if async_auto_reset_event::set is called by startWaiter while some other startAwaiter coroutine is already awaiting on the event.

It's also possible to achieve strict nesting - for example, by activating startSignaller after all startWaiter calls are suspended in the co_await event. If the number of waiters is big enough, then the failure is likely to occur in all test configurations.