Blocked: Replace `wstring` with `u16string`

The most correct C++ type for UTF-16 data is std::u16string, but due to a known bug in MSVC, the std::codecvt_utf8_utf16 converter cannot be used with char16_t / u16string types, so we must use wchar_t / wstring types for now.

I am deeply sorry for the statement I made about the availability of this fix. This is not fixed in Visual Studio 2017.

This issue requires a binary breaking change to the VC++ libraries, so we cannot ship this fix in an update to the libraries. Visual Studio 2017 shipped with v141 of the libraries, which is a minor update and binary compatible with v140 (the version shipped with Visual Studio 2015).

This bug will be fixed in the next major version of the Visual C++ Libraries.

Thanks,
Steve Wishnousky
Software Engineer II - Visual C++ Libraries
stwish@microsoft.com