Speed up string-lessp for multibyte strings

Speed up string-lessp for multibyte strings

Improve comparison speed when both arguments are multibyte strings,
at least one of them containing a non-ASCII character.  (All-ASCII
multibyte strings are already fast.)
The speed-up is about 2× for strings of 10 chars, 10× for strings of
100 chars.

* src/fns.c (Fstring_lessp): Quickly skip the common prefix by
comparing words.