Fix up planner infrastructure to support LATERAL properly.

This patch takes care of a number of problems having to do with failure
to choose valid join orders and incorrect handling of lateral references
pulled up from subqueries.  Notable changes:

* Add a LateralJoinInfo data structure similar to SpecialJoinInfo, to
represent join ordering constraints created by lateral references.
(I first considered extending the SpecialJoinInfo structure, but the
semantics are different enough that a separate data structure seems
better.)  Extend join_is_legal() and related functions to prevent trying
to form unworkable joins, and to ensure that we will consider joins that
satisfy lateral references even if the joins would be clauseless.

* Fill in the infrastructure needed for the last few types of relation scan
paths to support parameterization.  We'd have wanted this eventually
anyway, but it is necessary now because a relation that gets pulled up out
of a UNION ALL subquery may acquire a reltargetlist containing lateral
references, meaning that its paths *have* to be parameterized whether or
not we have any code that can push join quals down into the scan.

* Compute data about lateral references early in query_planner(), and save
in RelOptInfo nodes, to avoid repetitive calculations later.

* Assorted corner-case bug fixes.

There's probably still some bugs left, but this is a lot closer to being
real than it was before.

src/backend/nodes/equalfuncs.c

@@ -862,6 +862,15 @@ _equalSpecialJoinInfo(const SpecialJoinInfo *a, const SpecialJoinInfo *b)
 	return true;
 }
 
+static bool
+_equalLateralJoinInfo(const LateralJoinInfo *a, const LateralJoinInfo *b)
+{
+	COMPARE_SCALAR_FIELD(lateral_rhs);
+	COMPARE_BITMAPSET_FIELD(lateral_lhs);
+
+	return true;
+}
+
 static bool
 _equalAppendRelInfo(const AppendRelInfo *a, const AppendRelInfo *b)
 {
@@ -2646,6 +2655,9 @@ 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;