Write Visual Studio visualizations using C++

The current visualization of STL uses the built-in XML dialect, which is inflexible. The debugger actually has an API https://github.com/Microsoft/ConcordExtensibilitySamples/ that allows types to be visualized programmatically. Perhaps STL could consider adopting it. A complete example is https://github.com/microsoft/cppwinrt/tree/master/natvis.