Support Promise v3 #43

We should support Promise v3 for Async v2, v3 and v4. I don't think implementing this here requires a lot of work, the only outstanding feature that would be debatable is cancellation semantics as discussed in #42.

I'm not currently working on this, but figured it makes sense to report here in order to track this feature request. In case anybody feels like picking this up, any input and PRs would be much appreciated 👍