More string-search optimisations
All-ASCII strings cannot have substrings with non-ASCII characters in them; use this fact to avoid searching entirely. * src/fns.c (Fstring_search): For multibyte non-ASCII needle and unibyte haystack, don't check if the haystack is all-ASCII; it's a waste of time. For multibyte non-ASCII needle and multibyte all-ASCII haystack, fail immediately. * test/src/fns-tests.el (string-search): Add more test cases.