Before I place <experimental/filesystem> into cryostasis, there are a couple of things that I want to clean up, in case we ever need to look at it again while trying to understand legacy code:

• _FS_DLL => extern "C" _CRTIMP2_PURE
  • This macro obscured how we're dllexporting the separately compiled machinery. Nothing else in the STL used this pattern. It was also never overridden, so the #ifndef _FS_DLL was pointless.
• _Pchar => wchar_t
  • This alias obscured the path's character type, and wasn't consistently used.