Promises and futures largely compatible with the clojure.core api.

The main difference is, that [[cljs.core/deref]]/`@` cannot be used
on promises and futures. Instead [[async-deref]] can be used to work
with the respective results asynchronously. Also, futures may do
asynchronous operations, by returning a [[core/promise]].

Note that [[cljs.core/realized?]] works on both promises and futures.