(CLJS) js/Promise interop via core.async.interop

This issue follows up on a discussion about core.async.interop 31/10/2019 on Clojurians #clojurescript channel where it was noted that questions about promises were a recurring topic.

David Nolen provided a macro and remarked that it could be a candidate for the core.async.interop namespace along with a post on the topic.

I put up https://github.com/filipesilva/async-interop with the macro, some tests, and what I imagine could be a guide in https://clojurescript.org/guides/ (except for this "Notes" section).

The guide page can be found at , and attached is a patch for core.async.