Function: mm-string-to-multibyte

Return a multibyte string with the same individual chars as STRING.
If STRING is multibyte, the result is STRING itself.
Otherwise it is a newly created string, with no text properties.

If STRING is unibyte and contains an 8-bit byte, it is converted to
the corresponding multibyte character of charset `eight-bit'.

This differs from `string-as-multibyte' by converting each byte of a correct
utf-8 sequence to an eight-bit character, not just bytes that don't form a correct sequence. (fn STRING)