Get rid of the planner's LateralJoinInfo data structure.

I originally modeled this data structure on SpecialJoinInfo, but after
commit acfcd45cac that looks like a pretty poor decision.
All we really need is relid sets identifying laterally-referenced rels;
and most of the time, what we want to know about includes indirect lateral
references, a case the LateralJoinInfo data was unsuited to compute with
any efficiency.  The previous commit redefined RelOptInfo.lateral_relids
as the transitive closure of lateral references, so that it easily supports
checking indirect references.  For the places where we really do want just
direct references, add a new RelOptInfo field direct_lateral_relids, which
is easily set up as a copy of lateral_relids before we perform the
transitive closure calculation.  Then we can just drop lateral_info_list
and LateralJoinInfo and the supporting code.  This makes the planner's
handling of lateral references noticeably more efficient, and shorter too.

Such a change can't be back-patched into stable branches for fear of
breaking extensions that might be looking at the planner's data structures;
but it seems not too late to push it into 9.5, so I've done so.

src/backend/nodes/equalfuncs.c

@@ -845,15 +845,6 @@ _equalSpecialJoinInfo(const SpecialJoinInfo *a, const SpecialJoinInfo *b)
 	return true;
 }
 
-static bool
-_equalLateralJoinInfo(const LateralJoinInfo *a, const LateralJoinInfo *b)
-{
-	COMPARE_BITMAPSET_FIELD(lateral_lhs);
-	COMPARE_BITMAPSET_FIELD(lateral_rhs);
-
-	return true;
-}
-
 static bool
 _equalAppendRelInfo(const AppendRelInfo *a, const AppendRelInfo *b)
 {
@@ -2860,9 +2851,6 @@ equal(const void *a, const void *b)
 		case T_SpecialJoinInfo:
 			retval = _equalSpecialJoinInfo(a, b);
 			break;
-		case T_LateralJoinInfo:
-			retval = _equalLateralJoinInfo(a, b);
-			break;
 		case T_AppendRelInfo:
 			retval = _equalAppendRelInfo(a, b);
 			break;