Redo postgres_fdw's planner code so it can handle parameterized paths.

I wasn't going to ship this without having at least some example of how
to do that.  This version isn't terribly bright; in particular it won't
consider any combinations of multiple join clauses.  Given the cost of
executing a remote EXPLAIN, I'm not sure we want to be very aggressive
about doing that, anyway.

In support of this, refactor generate_implied_equalities_for_indexcol
so that it can be used to extract equivalence clauses that aren't
necessarily tied to an index.

src/include/optimizer/paths.h

@@ -49,9 +49,6 @@ extern List *generate_bitmap_or_paths(PlannerInfo *root, RelOptInfo *rel,
 extern bool relation_has_unique_index_for(PlannerInfo *root, RelOptInfo *rel,
 							  List *restrictlist,
 							  List *exprlist, List *oprlist);
-extern bool eclass_member_matches_indexcol(EquivalenceClass *ec,
-							   EquivalenceMember *em,
-							   IndexOptInfo *index, int indexcol);
 extern bool match_index_to_operand(Node *operand, int indexcol,
 					   IndexOptInfo *index);
 extern void expand_indexqual_conditions(IndexOptInfo *index,
@@ -99,6 +96,12 @@ extern bool have_join_order_restriction(PlannerInfo *root,
  * equivclass.c
  *	  routines for managing EquivalenceClasses
  */
+typedef bool (*ec_matches_callback_type) (PlannerInfo *root,
+													  RelOptInfo *rel,
+													  EquivalenceClass *ec,
+													  EquivalenceMember *em,
+													  void *arg);
+
 extern bool process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 					bool below_outer_join);
 extern Expr *canonicalize_ec_expression(Expr *expr,
@@ -126,10 +129,11 @@ extern void mutate_eclass_expressions(PlannerInfo *root,
 						  Node *(*mutator) (),
 						  void *context,
 						  bool include_child_exprs);
-extern List *generate_implied_equalities_for_indexcol(PlannerInfo *root,
-										 IndexOptInfo *index,
-										 int indexcol,
-										 Relids prohibited_rels);
+extern List *generate_implied_equalities_for_column(PlannerInfo *root,
+									   RelOptInfo *rel,
+									   ec_matches_callback_type callback,
+									   void *callback_arg,
+									   Relids prohibited_rels);
 extern bool have_relevant_eclass_joinclause(PlannerInfo *root,
 								RelOptInfo *rel1, RelOptInfo *rel2);
 extern bool has_relevant_eclass_joinclause(PlannerInfo *root,