fixes developit#35

tldr; Mainly wrapped the existing promise api to get it to work
with async generators.
I took atleast two iterations to get to this point. At first I thought
to do the job with a readable Stream implementing async iterable
on the main thread, but then was afraid of inconsistencies that
would arise between the two apis. For example Readable Stream
when finished will only return `{ done: true, value: undefined }`
whereas async iterables can return `{ done: true, value: any }`
when `any` is any value.

So, then I decided to make a async generator that could talk to
the worker for better compatibility. One thing to note is that the worker
data onmessage receives an extra piece for the status to cause the
iterator to use. This is similar to the Promise status, but for
generators.