Fix async iterator flatmap not flatmapping async iterables

[Jamesernator]

This replaces #57 by introducing a new operation GetIteratorOrThrow and replaces the existing GetIterator with one that does no throw if the value is not iterable.

[zloirock]

@Jamesernator could you also add a fix from this decision?

[devsnek]

i am planning to leave this open until 55 is resolved one way or another. until then, its probably best to keep the discussion to 55.

[zloirock]

@devsnek hey, without this fix, flatMap is completely broken and we need proposed abstract operations anyway. Maybe better to merge it for making this method in the spec to work at least somehow and after resolving of #55 change it if it will be required?

[devsnek]

Async methods are also broken because they don't handle errors correctly yet. I'd like to worry about our intention first, and then we can worry about how to spec our intention.

[zloirock]

Yep, but it's 2 different spec issues -) The code from this PR could be reused after any decision on #55.

[bakkot]

I believe this is fixed as of #59.

[devsnek]

yep.