full revert on the idea not to allow spaces in PGQ arrow specifications

now, we allow them

so the <-> and <- symbols were removed again.

This change of thought was triggered by the mayhem the new lexical
symbols were causing:

================================================================================
Query unexpectedly failed (test/optimizer/move_constants.test:101)
================================================================================
EXPLAIN SELECT -1*X<-5 FROM test;
================================================================================
Actual result:
Parser Error: syntax error at or near "<-"
LINE 1: EXPLAIN SELECT -1*X<-5 FROM test

but the main result of this PR is still: you can now type ->* without spaces

third_party/libpg_query/grammar/grammar.y

@@ -108,7 +108,7 @@
 %nonassoc	SET				/* see */
 %left		UNION EXCEPT
 %left		INTERSECT
-%left		LAMBDA_ARROW DOUBLE_ARROW ARROW_LEFT ARROW_BOTH
+%left		LAMBDA_ARROW DOUBLE_ARROW
 %left		OR
 %left		AND
 %right		NOT

third_party/libpg_query/grammar/statements/pgq.y

@@ -757,18 +757,24 @@ FullElementSpec:
 			}
 		;
 
-/* we now do *not* allow spaces inside the arrows (we extended the DDB lexer in scan.l and grammar.y for this) */
+/* we allow spaces inside the arrows */
 Arrow:
         '-'
             {   $$ = "-"; }
     |
-        ARROW_LEFT
+        '<' '-'
             {   $$ = "<-";  }
     |
         LAMBDA_ARROW
             {   $$ = "->"; }
     |
-        ARROW_BOTH
+        '-' '>'
+            {   $$ = "->"; }
+    |
+        '<' LAMBDA_ARROW
+            {    $$ = "<->";  }
+    |
+        '<' '-' '>'
             {    $$ = "<->";  }
     |
         Op
@@ -793,7 +799,7 @@ ArrowLeft:
         '-' '['
             {   $$ = "-"; }
     |
-        ARROW_LEFT '['
+        '<' '-' '['
             {   $$ = "<-";  }
         ;
 

third_party/libpg_query/scan.l

@@ -333,8 +333,6 @@ dot_dot			\.\.
 colon_equals	":="
 lambda_arrow	"->"
 double_arrow    "->>"
-arrow_left      "<-"
-arrow_both      "<->"
 power_of        "**"
 integer_div     "//"
 
@@ -828,16 +826,6 @@ other			.
 					return LAMBDA_ARROW;
 				}
 
-{arrow_left}	{
-					SET_YYLLOC();
-					return ARROW_LEFT;
-				}
-
-{arrow_both}	{
-					SET_YYLLOC();
-					return ARROW_BOTH;
-				}
-
 {double_arrow}  {
                     SET_YYLLOC();
                     return DOUBLE_ARROW;