impl/base.h is not namespaced

[dlech]

When you add the winmd package to your include directories and your project already contains it's own "impl/base.h" header file, it causes problems.

This could be solved by moving everything in impl up a directory level and removing the impl folder and renaming base.h to winmd_base.h.

[kennykerr]

Note that this winmd parser is no longer under active development and mostly serves as an implementation detail of https://github.com/microsoft/cppwinrt so I'm reluctant to make any significant changes at this point.