Support LIKE and ILIKE index searches via contrib/pg_trgm indexes.

Unlike Btree-based LIKE optimization, this works for non-left-anchored
search patterns.  The effectiveness of the search depends on how many
trigrams can be extracted from the pattern.  (The worst case, with no
trigrams, degrades to a full-table scan, so this isn't a panacea.  But
it can be very useful.)

Alexander Korotkov, reviewed by Jan Urbanski