String Theory 2.3

• Fix char8_t and std::u8string detection when the compiler supports it but the selected C++ standard does not.
• Fix a potential infinite loop when substituting bad UTF-8 sequences in ST::string.
• Add c_str() methods to ST::buffer<T>.
• Add _stbuf user literal operator and ST_(CHAR|WCHAR|UTF16|UTF32)_LITERAL macros.
• Improve performance of ST::format and friends when formatting floating point values.
• Apply forwarding of ST::format and friends format arguments to reduce copying.