https://en.cppreference.com/w/cpp/header/codecvt
/usr/include/c++/<gcc-ver>/codecvt

codecvt_utf8_utf16

https://en.cppreference.com/w/cpp/locale/wstring_convert/converted
https://en.cppreference.com/w/cpp/locale/codecvt_utf8

qa
https://stackoverflow.com/questions/2573834/c-convert-string-or-char-to-wstring-or-wchar-t
https://stackoverflow.com/questions/17103925/how-well-is-unicode-supported-in-c11
http://qaru.site/questions/300873/converting-c-stdwstring-to-utf8-with-stdcodecvtxxx
http://qaru.site/questions/42416/how-to-convert-wstring-into-string

compiler support
https://stackoverflow.com/questions/15615136/is-codecvt-not-a-std-header

deprecation
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0618r0.html

boost locale
https://www.boost.org/doc/libs/develop/libs/locale/doc/html/files.html
https://www.boost.org/doc/libs/develop/libs/locale/doc/html/encoding__utf_8hpp_source.html

clang locale
https://www.linux.org.ru/forum/development/11400521