- add the convertion between yycc_char8_t and system char type because we decide use our char8_t in the whole library.
- make a clear boundary between yycc char8_t declarations and related assist functions. the declarations present in internal header and assist functions are written in encoding helper.
- use std::basic_string_view instead of std::basic_string to provide more abilities to encoding convertion functions and reduce the redundant memory occupation at the same time.
- 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.
