bug#13041: 24.2; diacritic-fold-search

[Drew Adams]

FWIW - Some more browsing on the topic tells me that what we are trying to come
up with here is a predicate for the NFKD canonical ordering (as applied to a
char sequence, not to a single char).

IOW, a string-ordering predicate that uses the canonical ordering for a
character's decomposed normal code point sequence.

We are using compatibility normalization, not canonical normalization.  So a
search (or a string comparison test) for `f' will match the ligature `ffi'
(whereas it would not match wrt canonical normalization).

Someone please correct me if any of this is wrong.