Provide to_hstring(IStringable const&) overload

[jaigak]

Provide an overload of to_hstring for IStringable that simply returns IStringable::ToString. This will unify the stringification of WinRT classes and primitive types. This is also useful for XAML since it's only aware of to_hstring and not IStringable::ToString when using from C++/WinRT.

[sylveon]

Where does XAML use to_hstring?

[jaigak]

@sylveon When binding to something that's not a string to a string.

[kennykerr]

Seems like a reasonable addition.

[kennykerr]

Of course, I don't know whether that would actually be used by Xaml without changes to Xaml - you'd have to experiment first to see whether that's effective. The overload of to_hstring could also not appear beside the other overloads since it depends on Windows.Foundation.

[sylveon]

I suppose a lot of XAML wouldn't work without Windows.Foundation anyways :P

[github-actions]

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