Implement the standard string library functions

• Implement strstr() (or some other string library function).

   strlen()    Get length of a string.
   strcpy()    Copy one string to another.
   strcat()    Link together (concatenate) two strings.
   strcmp()    Compare two strings.
   strchr()    Find character in string.
   strstr()    Find string in string.
   strlwr()    Convert string to lowercase.
   strupr()    Convert string to uppercase.