IN clauses appearing at top level of WHERE can now be handled as joins.

There are two implementation techniques: the executor understands a new
JOIN_IN jointype, which emits at most one matching row per left-hand row,
or the result of the IN's sub-select can be fed through a DISTINCT filter
and then joined as an ordinary relation.
Along the way, some minor code cleanup in the optimizer; notably, break
out most of the jointree-rearrangement preprocessing in planner.c and
put it in a new file prep/prepjointree.c.

src/test/regress/expected/subselect.out

@@ -58,10 +58,10 @@ SELECT '' AS six, f1 AS "Uncorrelated Field" FROM SUBSELECT_TBL
  six | Uncorrelated Field 
 -----+--------------------
      |                  1
-     |                  2
-     |                  3
      |                  1
      |                  2
+     |                  2
+     |                  3
      |                  3
 (6 rows)
 
@@ -71,10 +71,10 @@ SELECT '' AS six, f1 AS "Uncorrelated Field" FROM SUBSELECT_TBL
  six | Uncorrelated Field 
 -----+--------------------
      |                  1
-     |                  2
-     |                  3
      |                  1
      |                  2
+     |                  2
+     |                  3
      |                  3
 (6 rows)
 
@@ -134,10 +134,10 @@ SELECT '' AS five, f1 AS "Correlated Field"
                      WHERE f3 IS NOT NULL);
  five | Correlated Field 
 ------+------------------
-      |                2
-      |                3
       |                1
       |                2
+      |                2
+      |                3
       |                3
 (5 rows)