Faster `string-lessp` for unibyte arguments

Faster `string-lessp` for unibyte arguments

Since this function is commonly used as a sorting predicate
where it is time-critical, this is a useful optimisation.

* src/fns.c (Fstring_lessp): Add fast path for the common case
when both arguments are unibyte.
* test/src/fns-tests.el (fns-tests--string-lessp-cases)
(fns-tests-string-lessp): New test.