Unify Await with variant used in AsyncGeneratorResumeNext

[schuay]

AsyncGeneratorResumeNext [0] uses a minor variant of Await [1] with different onFulfilled and onReject closures, but that is not clear just by looking at steps 10.b.i.2-9 (at least not to me).

WDYT about either extracting these steps into a new named AsyncGeneratorAwait function (or similar), or refactoring Await to take onFulfilled and onRejected as arguments?

[0] https://tc39.github.io/proposal-async-iteration/#sec-asyncgeneratorresumenext
[1] https://tc39.github.io/proposal-async-iteration/#await

[domenic]

I guess we could, but I'd kind of rather wait for tc39/ecma262#933 to clean this up entirely. But I'd be happy to take a pull request refactoring it in the meantime.