Add option to strip our rarely used C++/WinRT templates and traits

[kennykerr]

There are a few areas where C++/WinRT provides "helpful" templates and traits, but some of these are rarely used and add a significant expense to compilation time. They cannot simply be removed without potentially breaking existing code. For now, I'm providing the option to remove them by defining WINRT_LEAN_AND_MEAN. If subsequent testing reveals that this can be broadly applied then this may be "turned on" for everyone at some future point. Defining WINRT_LEAN_AND_MEAN impacts three areas.

1. Variable templates are used to make winrt::name_of<T>() work with all WinRT types while WinRT itself only requires the names of WinRT classes, structs, and enums. This leaves a huge amount of WinRT interface and delegate names that go unused. Defining WINRT_LEAN_AND_MEAN will strip out the variable template specializations for WinRT interfaces and delegates.

2. std::hash<T> specializations are provided for all WinRT classes and interfaces on the assumption that someone might want to use such types as the key for a std::unordered_map<K, V>. However, this is rarely used and developers can easily provide a hashing function for the odd type they actually use. Defining WINRT_LEAN_AND_MEAN will strip out the specializations for WinRT classes and interfaces.

3. C++/WinRT allows any WinRT interface to be used or called as well as to be implemented. Some WinRT interfaces are however exclusive to a given WinRT class and should not be implemented by any other object. C++/WinRT allowed this to accommodate implementing WinRT classes, but it leaves a great many internal class template specializations defined that hardly any project using C++/WinRT will ever implement. Defining WINRT_LEAN_AND_MEAN will strip out the specializations of exclusive interfaces that are not implemented by the project. Non-exclusive interfaces remain available for all to implement.

Defining WINRT_LEAN_AND_MEAN shaves more than 10% off the size of my large test PCH. Again, this is an opt-in for the moment. To receive this benefit you need to define WINRT_LEAN_AND_MEAN before including any C++/WinRT headers.

Fixes #612