Force-stopping a coroutine

[nelsonxb]

The documentation currently states that while green_await(NULL) is a really bad plan, it may be used in future to exit early from a thread (and clean it up).

This does honestly sound incredibly dodgy to me, but I can't shake the feeling that it might end up being necessary in some weird use-case.

[nelsonxb]

Although... just because we might be able to clean up the coroutine stack properly, doesn't mean resource leaks are avoided. User code may have all sorts of resources held, and at least by specifically documenting this as a Bad Idea All Around, we're discouraging behaviour that may lead to leakage for the careless, meanwhile explicitly supporting that use-case may create a false sense of security.

Maybe the correct answer to force-cancellation is C++ exceptions.