add Async::Promise

[paddor]

This is a draft to fix #370

[ioquatix]

• Missing documentation for reject and rejected?.

On the surface, I'm okay with this change. However, I am not sure I want to merge it yet.

I wonder if resolved? should return true if the variable was rejected. At least, that feels more natural to me e.g. from a usage point of view. The point of such a method is to control level signalling, e.g. https://github.com/socketry/async-http/blob/e2598452c5d014623cee16b451f1cb84e4e74f27/lib/async/http/protocol/http1/finishable.rb#L18

I wonder if we should encourage users just to use Async in the case they need a full-blown promise style interface.

Maybe the code above can be simplified with this change.

I would like to explore this further.

[paddor]

I've extracted the feature into Async::Promise. If necessary, we could release this in another gem.