This MR adds an implementation for the `IAsyncAction` and `IAsyncInfo` interfaces exported by `Windows.System.Threading`, including support for setting `IAsyncActionCompletedHandler` values for an `IAsyncAction`. It also re-writes (or factors?) the `IAsyncOperation<boolean>` implementation in dlls/cryptowinrt to use `ThreadPoolStatics::RunAsync`, by introducing a type-punning variant of `AsyncOperationCompletedHandler<T>` (similar to `WineAsyncOperationCompletedHandler`), which is a wrapper around the `IAsyncAction` returned by `RunAsync`.
The new `IAsyncOperation<T>` implementation is simpler, and if merged, can be exported to other dlls (windows.gaming.input and windows.security.credentials.ui.userconsentverifier for now).
-- v5: cryptowinrt: Rewrite IAsyncOperation using Windows.System.Threading.