Use BreakIterator for search_coll and search_fixed (optionally)

[gagolews]

usearch_open (const UChar *pattern, int32_t patternlength, const UChar *text, int32_t textlength, const char *locale, UBreakIterator *breakiter, UErrorCode *status) -- a BreakIterator may be provided to search only @ specific text boundaries (e.g. match whole words only)

allow search funs to be fed with a break iterator to use; add briter support for search_fixed

[gagolews]

• [x] #110 introduce stri_opts_fixed.
• [x] #111 allows opts to be passed via ...
• [x] These functions will have to silently ignore unknown opts.

here we'll need a combination of stri_opts_brkiter and stri_opts_fixed/stri_opts_coll