Async iterators

[Clinton Gormley]

Hi Marc

> 2) change your api to be event-driven, by always returning "instantly" and
>    instead calling a callback with results.

This is what I'm hoping to get to. Just struggling to visualize what the
code and usage actually looks like.

I think I'm going to throw away what I've got and try to write it fresh,
keeping the "don't call us, we'll call you" mantra in mind.

I'll ping back for opinions when I've got something to look at.

many thanks

Clint