Correct me if I'm wrong.

I read cod eand found definion of Lisp_String:
  struct GCALIGNED Lisp_String
  {
    ptrdiff_t size;
    ptrdiff_t size_byte;
    INTERVAL intervals;		/* Text properties in this string.  */
    unsigned char *data;
  };

I understand string text is encoded in UTF8 format and is stored in
'Lisp_String::data'. There is actually no difference between unibyte
and multibyte text since UTF8 is compatible with ASCII and we only deal
with 'data' field.

I attached the latest patch.