Non-blocking poll_list alternative

[guybedford]

In JS hosts, implementing poll_list as blocking requires some asyncify work.

It could be nice to have a non-blocking alternative to poll_list that could be used as part of an actual polling scheme, if that kind of a model would be worth considering.

Just being able to inspect if a poll is ready should be enough to support this.

[guybedford]

pollable.ready() resolves this question.