New supported listener return values: function, async Iterator

[deanrad]

• A function. This is the simplest way to get a deferred value, so that serial mode could work for example.
• An async iterator/generator Despite their syntax being forboding IMHO, they are part of the language, and should probably work if cast to Observables internally. More info https://javascript.info/async-iterators-generators

Async iterator is the hard part though..