Experimentally Translate Async Generator Functions

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Async generator functions should produce, when called, an async generators. Async generators have the same methods as regular generators (next, throw, and return), but each method returns a promise for an InteratorResult.

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I believe this can be achieved by translating await calls into yields where the yield argument is an instance of a magic class.

We'll have an async runner which resembles the runner used for regular async functions. When the generator yields an instance of the magic class, then we resolve-and-feed in the normal async runner style. Otherwise, we enqueue the result in an output queue.

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Initial work complete in 618b8a89248b8645c5a7d099414fcd70721f344d