Implement AsyncSequence.enumerated()

[adam-fowler]

Pretty much the same as Sequence.enumerated but for AsyncSequence.

eg Translates ["a", "b", "c"] to [(0,"a"), (1, "b"), (2, "c")]

[FranzBusch]

Thanks for opening this @adam-fowler! Definitely a needed AsyncSequence. We are still finalising the 1.0.0 release. I will mark this as a 1.1.0 candidate