Async-streams: Add support for enumerator-returning async-iterator method

[jcouv]

async System.Collections.Generic.IAsyncEnumerator<string> M() { ... }

Some notes to self:

• Will probably need to override InitializeStateMachine for async-iterator state machines. See the comment in that method for iterator state machines:

            // var stateMachineLocal = new IteratorImplementationClass(N)
            // where N is either 0 (if we're producing an enumerator) or -2 (if we're producing an enumerable)

• The kickoff method probably needs some other changes too

• Will probably need to modify PreserveInitialParameterValues (true for IAsyncEnumerable, but false for IAsyncEnumerator or regular async methods)

• Some methods can be removed (GetAsyncEnumerator, DisposeAsync)

• Some interfaces can be removed (IAsyncEnumerable, IAsyncDisposable)