Phrase full text search.

Patch introduces new text search operator (<-> or <DISTANCE>) into tsquery.
On-disk and binary in/out format of tsquery are backward compatible.
It has two side effect:
- change order for tsquery, so, users, who has a btree index over tsquery,
  should reindex it
- less number of parenthesis in tsquery output, and tsquery becomes more
  readable

Authors: Teodor Sigaev, Oleg Bartunov, Dmitry Ivanov
Reviewers: Alexander Korotkov, Artur Zakirov

src/backend/utils/adt/tsvector.c

@@ -28,7 +28,7 @@ typedef struct
 
 
 /* Compare two WordEntryPos values for qsort */
-static int
+int
 comparePos(const void *a, const void *b)
 {
 	int			apos = WEP_GETPOS(*(const WordEntryPos *) a);