Releasing owned resources on Generator drop

[noncrab]

I notice that there's no Drop implementation for Generator, and so it looks like it's owned stack won't get unwound, and so any resources owned by a Generator's stack will get leaked.

So, it seems that that the only way currently to forcefully unwind a generator's stack is to pass in a value that asks it to panic, such as in this example. Is that right?

I notice that context-rs has the ability to spawn a function on top of a recovered stack, which might be an option for future implementation. But that seems quite risky.

[Amanieu]

See #52