[nowide] Convert wstring to u8string (C++20)

From: Jim Orcheson (jimorcheson_at_[hidden])
Date: 2021-07-01 16:56:20

In C++17 and earlier, nowide::narrow can be used to convert from UTF-16 
encoded strings (std::wstring) to UTF-8 encoded strings, stored in 
std::string. C++20 introduced std::u8string for UTF-8 encoded strings 
and it is incompatible with std::string. Is there some way I can get 
from std::wstring to std::u8string? It is not at all obvious from the 
nowide documentation.