refactor: refactor encoding helper again.

- 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.

src/YYCCInternal.hpp

@@ -25,22 +25,19 @@
 #endif
 
 // Define the UTF8 char type we used.
-// Also define an universal macro to create UTF8 string literal.
 // And do a polyfill if no embedded char8_t type.
 #include <string>
+#include <string_view>
 namespace YYCC {
 #if defined(__cpp_char8_t)
 	using yycc_char8_t = char8_t;
 	using yycc_u8string = std::u8string;
-
-#define _YYCC_U8(strl) u8 ## strl
-#define YYCC_U8(strl) (_YYCC_U8(strl))
+	using yycc_u8string_view = std::u8string_view;
 #else
 	using yycc_char8_t = unsigned char;
 	using yycc_u8string = std::basic_string<yycc_char8_t>;
-
-#define _YYCC_U8(strl) u8 ## strl
-#define YYCC_U8(strl) (reinterpret_cast<const yycc_char8_t*>(_YYCC_U8(strl)))
+	using yycc_u8string_view = std::basic_string_view<yycc_char8_t>;
 #endif
+	
 }