add string splitting functions

[neoncitylights]

General:

char *strSplitAt(const char *str, size_t i);
char *strSplitAtMut(char *str, size_t i);

By character:

char **strSplitChar(const char *str, char c);
char **strSplitCharN(const char *str, char c, size_t times);

char **strSplitCharMut(char *str, char c);
char **strSplitCharNMut(char *str, char c, size_t times);

By string:

char **strSplitStr(const char *str, const char *substr);
char **strSplitStrN(const char *str, const char *substr, size_t times);

char **strSplitStrMut(char *str, const char *substr);
char **strSplitStrNMut(char *str, const char *substr, size_t times);