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_parser.c

@@ -89,7 +89,15 @@ do { \
 	} \
 } while (0)
 
-#define ISOPERATOR(x)	( pg_mblen(x)==1 && ( *(x)=='!' || *(x)=='&' || *(x)=='|' || *(x)=='(' || *(x)==')' ) )
+/* phrase operator begins with '<' */
+#define ISOPERATOR(x) \
+	( pg_mblen(x) == 1 && ( *(x) == '!' ||	\
+							*(x) == '&' ||	\
+							*(x) == '|' ||	\
+							*(x) == '(' ||	\
+							*(x) == ')' ||	\
+							*(x) == '<'		\
+						  ) )
 
 /* Fills gettoken_tsvector's output parameters, and returns true */
 #define RETURN_TOKEN \