Add synchronous generator support for IIterable<T>

[Fulgen301]

This implements #1278.

IIterable::First() throws E_CHANGED_STATE if it is called more than one time since the coroutine has already been resumed at that point; however, I am unsure whether this violates the API contract:

If changes are made to the collection, such as adding, modifying, or deleting elements, the iterator is permitted to raise an exception for all future operations.

[Fulgen301]

@microsoft-github-policy-service agree

[kennykerr]

Unfortunately, a project maintainer is not currently available to review this pull request. Please see the contributing guide for more information. Feel free to keep the conversation going on the related issue.

[kennykerr]

The https://github.com/microsoft/wil library is where we've been collecting implementation helpers such as this. You might want to open an issue on that repo for consideration.

[jonwis]

The https://github.com/microsoft/wil library is where we've been collecting implementation helpers such as this. You might want to open an issue on that repo for consideration.

Yes - this looks much more appropriate for wil or https://github.com/microsoft/cpp-async as it doesn't really require cppwinrt on its own. Looking forward to the PR on either of those places!

[sylveon]

Feels weird to have to IAsyncOperation support be built-in but not IIterable. Perhaps both should be moved to wil?

[kennykerr]

Async coroutine support is fundamental to C++/WinRT for both consumption and production. Generator support for iteration, while cool, has very narrowly defined application.