Support for C++20 u8string

[Apivan]

Since WString already stored utf8_ as std::string. This is just additional interfaces for accepting std::u8string arguments. No conversion needed.

https://en.cppreference.com/w/cpp/language/types#char8_t

[emweb]

Thanks for your contribution.

Ideally Wt would use u8string as its internal representation and use it throughout, but maybe that's more of a Wt 5 thing (we don't have plans for another major release anytime soon).

I have a few remarks:

• Binary compatibility: if you're using C++20, but Wt was compiled with C++17, this will cause linker errors when the u8string functions are used. I suggest perhaps we make the char8_t/std::u8string functions inline. We do already have an accidental incompatibility due to the date library using std::string_view instead of std::string with C++17, but let's try not to introduce new incompatibilities.
• We need unit tests for these functions.
• The operator std::u8string() function is not implemented correctly, consider localized WStrings or WStrings with arguments. You'd have to construct a u8string from the result of toUTF8().
• The #ifdefs should probably contain the documentation. I think if we want to generate proper documentation, we'll need to check #if defined(__cpp_char8_t) || defined(DOXYGEN_ONLY). Use the doxygen target to verify that the documentation is generated properly.
• The WString(const std::u8string&) constructor lacks a doc comment.

Regards,
Roel

[Apivan]

Thank you for your reply. I believe that using std::u8string internally in the future Wt would be a better solution. Initially, I attempted to migrate our workplace's project source code to u8string in order to solve some issues, but I then realized that most third-party libraries do not support this. Changing large project to u8string would be a massive undertaking and cause excessive copying and conversion . So I withdraw this pull request.