Implement katze_utf8_stristr, katze_collfold and use in completion

Implement katze_utf8_stristr, katze_collfold and use in completion

Evidently normalizing any whole string is too slow for completion,
so we need to even out case and composition while iterating through
the strings.

The decompositing version of katze_utf8_stristr is disabled, since
it is too slow, and an ascii only version is used, for now.

A unit test 'compare' is added that solely measures performance of
katze_collfold and katze_utf8_stristr.