- add yycc_char8_t and yycc_u8string in code to indicate explicit utf8 char type and string. it also has a polyfill if compiler and library do not support utf8 char type.
- refactor the whole encoding helper. allow converting string with embedded NUL. but not tested.
- use std::codevct as the convertion method among UTF8, UTF16 and UTF32.
- fix the issue that COM Guard was accidently dropped by compiler because no reference to it.
