make PGQ arrows undivisble and remove the need for a spece between ->…

… and *

- add new lexical symbols for <- and <->
- reshape the pgq.y grammar rules to use these

third_party/libpg_query/grammar/grammar.y

@@ -79,7 +79,7 @@
 %token <str>	IDENT FCONST SCONST BCONST XCONST Op
 %token <ival>	ICONST PARAM
 %token			TYPECAST DOT_DOT COLON_EQUALS EQUALS_GREATER INTEGER_DIVISION POWER_OF LAMBDA_ARROW DOUBLE_ARROW
-%token			LESS_EQUALS GREATER_EQUALS NOT_EQUALS
+%token			LESS_EQUALS GREATER_EQUALS NOT_EQUALS ARROW_LEFT ARROW_BOTH
 
 /*
  * If you want to make any keyword changes, update the keyword table in
@@ -108,7 +108,7 @@
 %nonassoc	SET				/* see */
 %left		UNION EXCEPT
 %left		INTERSECT
-%left		LAMBDA_ARROW DOUBLE_ARROW
+%left		LAMBDA_ARROW DOUBLE_ARROW ARROW_LEFT ARROW_BOTH
 %left		OR
 %left		AND
 %right		NOT