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 <[email protected]>

* 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 <[email protected]>

---------

Co-authored-by: Jon Skeet <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>
Co-authored-by: Nigel-Ecma <[email protected]>

Author: 4 people