Add C++/WinRT when_any async helper

[kennykerr]

Complements #516 (and improves upon #520) by adding a when_any helper function that will complete when any of the supplied async objects have completed. The resulting async object can then either be waited upon using co_await when_any(...) or when_any(...).get(). In addition, if the async objects are IAsyncOperation or IAsyncOperationWithProgress then the async object created by when_any will also return the result of the first async object to complete.

This is a little harder to implement than when_all, but just as useful for things like redundant operations or cooperative timeouts. As I mentioned in #341, WinRT only allows waiting once. That means that any objects that may not yet have completed may not be waited on again in any way.

Here's an example:

using namespace std::literals;
using namespace winrt;
using namespace Windows::Foundation;

IAsyncOperation<TimeSpan> wait_for(TimeSpan timeout)
{
    co_await timeout;
    printf("wait_for %lld\n", timeout.count());
    co_return timeout;
}

IAsyncAction test()
{
    co_await when_all(wait_for(3s), wait_for(2s), wait_for(1s));
    puts("* when_all returned");

    puts("");

    TimeSpan result = co_await when_any(wait_for(3s), wait_for(2s), wait_for(1s));
    printf("* when_any returned %lld\n", result.count());
}

int main()
{
    init_apartment();
    test().get();
    getchar();
}

And this program produces the following output:

wait_for 10000000
wait_for 20000000
wait_for 30000000
* when_all returned

wait_for 10000000
* when_any returned 10000000
wait_for 20000000
wait_for 30000000

Another example is to use it to implement a cooperative timeout:

template <typename T>
IAsyncOperation<T> get_default_after(T value, TimeSpan timeout)
{
    co_await timeout;
    co_return value;
}

For example:

IAsyncOperation<int> get_the_answer();

int main()
{
    init_apartment();

    int value = when_any(get_the_answer(), get_default_after(0, 2s)).get();
    printf("%d\n", value);
}

And this program will print 42 if it is able to compute it within 2 seconds. Otherwise it prints 0.