allow async iterator to be already aborted.

[martinheidegger]

I noticed that a signal can be aborted when being passed-in. This PR checks if the signal is aborted and throws an error.

[mafintosh]

Small comment, looks great.

@martinheidegger in general do you know if people unregister the abort listenener when your resource completes succesfully?

[martinheidegger]

in general do you know if people unregister the abort listenener when your resource completes succesfully?

I obviously didn't think of that, thanks @mafintosh for pointing this out. Naively I assumed that abortSignals and streams are usually short-lived objects that should be quickly garbage collected but indeed, if a async process (eg. in a cli application starting the server) can be a lot longer-lived it should be removed. - node.js of course removes the listener here - I added a commit that takes care of this!

[martinheidegger]

Rebased to recent code updates