promises

The migration guide here should be expanded with additional content regarding upgrading from Observable to ZStream.

I think having Crank emit errors/warnings in more situations would go a long ways towards compensating for the docs' lack of nuance.

E.g., an async generator returning undefined doesn't strike me as a common thing to want to do, and if someone wants it maybe they should return null so it's explicit. Then Crank could throw an error if an async generator returns undefined. That should