Here is a more polished version of the function pointer approach, now adapted to all multibyte encodings. Using the not-yet-committed tests from [1], I found a thinko bug that resulted in the test for nul bytes to not only be wrong, but probably also elided by the compiler. Doing it correctly is noticeably slower on pure ascii, but still several times faster than before, so the conclusions haven't changed any. I'll run full measurements later this week, but I'll share the patch now for review.
