Change methods/types from AsyncIterator to AsyncIterable

[stephentuso]

After opening #192, I looked into it more and saw that graphql-js is actually expecting an AsyncIterable, not AsyncIterator, to be returned from the subscribe function: source.

This PR updates the types, methods, and docs to reflect that. This is a breaking change. As mentioned in leebyron/iterall#49, TS can't recognize $$asyncIterator as Symbol.asyncIterator, so type assertions had to be used in a few places

[apollo-cla]

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[grantwwu]

So as I'm understanding it, AsyncIterables are objects which you can get an asyncIterator out of using the $$asyncIterator function. So technically our asyncIterators should always have been an asyncIterator in the first place?

So this is a breaking change in the sense that we're changing the types, but all working implementations should already be AsyncIterables, right?

[grantwwu]

Also, this seems like it's related to #147 ...

[stephentuso]

Oops - didn't do my homework on this one. Yes, this is basically the same as #147 without the changes re. #143. It is breaking because types and method names changed, you are correct though, it looks like all the implementations are already returning AsyncIterables.

Seems like I should probably just close this in favor of #147?

[Akxe]

How about this would look into implementing #223 (generic types) too?

[hwillson]

It looks like the work here has been superseded by #232 (which is coming in 3.0. Let us know if anything is missing, and sorry we didn't get to this contribution sooner!