Add support for Async Streams (#606)

* add support for async streams

* add support for async streams

* add support for async streams

* add example annotation

* Create Support1AsyncStreams.cs

* Create Support2AsyncStreams.cs

* Update standard/classes.md

Co-authored-by: Bill Wagner <wiwagn@microsoft.com>

* Add new types to C.5

* Split `foreach` and `await foreach` clauses

Add types to the standard library, and split the `foreach` and `await foreach` clauses. All three remaining clauses need further edits.

* Edit pass on enumerable objects.

* reorder async functions and iterators

* build fix

* Apply suggestions from code review

Co-authored-by: Nigel-Ecma <6654683+Nigel-Ecma@users.noreply.github.com>

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Co-authored-by: Jon Skeet <skeet@pobox.com>
Co-authored-by: Bill Wagner <wiwagn@microsoft.com>
Co-authored-by: Nigel-Ecma <6654683+Nigel-Ecma@users.noreply.github.com>

Author: 4 people