feat: Stream to AsyncIterator

[jpopesculian]

An attempt to implement the second part of #2399. I haven't written tests yet, but I wanted to have one of y'all take a look at it first before I invest more time into it. Of course, there's weirdness here because promises require 'static, but I figured since we're in a non-threaded context anyways it would be okay to just wrap the stream in an RefCell. However this does mean that there's no guarantees on order if you do something like

let iter = async_iterable_from_rust();
let out = Promise.all([iter.next(), iter.next()])

To counteract that, you could probably just create a chain of oneshot channels but I wanted to get your feedback on the RefCell stuff before I did any more work on it.