feat: add new split function reducing memory cost.

- add a new split function, SplitView which can reduce cost memory by using string view.
- add a new testbench for split function for testing empty source string.
- add documentation for some string helper function.
- improve library encoding documentation.

doc/src/string_helper.dox

@@ -3,4 +3,41 @@
 \page string_helper String Helper
 
 
+\section string_helper_lower_upper Lower Upper
+
+String helper provides Python-like string lower and upper function.
+Both lower and upper function have 2 overloads:
+
+\code
+yycc_u8string Lower(const yycc_char8_t*);
+void Lower(yycc_u8string&);
+\endcode
+
+First overload accepts a NULL-terminated string as argument and return a \b copy whose content are all the lower case of original string.
+Second overload accepts a mutable string container as argument and will make all characters stored in it become their lower case.
+You can choose on of them for your flavor and requirements.
+Upper also has similar 2 overloads.
+
+\section string_helper_split Split
+
+String helper provides Python-like string split function.
+It has 2 types for you:
+
+\code
+std::vector<yycc_u8string> Split(const yycc_u8string_view&, const yycc_char8_t*);
+std::vector<yycc_u8string_view> SplitView(const yycc_u8string_view&, const yycc_char8_t*);
+\endcode
+
+All these overloads take a string view as the first argument for the string need to be split.
+The second argument is a raw string pointer representing the decilmer for splitting.
+The only difference between these 2 split function are overt according to their names.
+The first split function will return a list of copied string as its split result.
+The second split function will return a list of string view as its split result,
+and it will keep valid as long as the life time of your given string view argument.
+It also means that the last type will cost less memory if you don't need the copy of original string.
+
+If the source string (the string need to be split) is empty, or the decilmer is \c nullptr or empty,
+the result will only has 1 item and this item is source string itself.
+There is no way that this method return an empty list, except the code is buggy.
+
 */