Restructure representation of aggregate functions so that they have pg_proc

entries, per pghackers discussion.  This fixes aggregates to live in
namespaces, and also simplifies/speeds up lookup in parse_func.c.
Also, add a 'proimplicit' flag to pg_proc that controls whether a type
coercion function may be invoked implicitly, or only explicitly.  The
current settings of these flags are more permissive than I would like,
but we will need to debate and refine the behavior; for now, I avoided
breaking regression tests as much as I could.

src/backend/utils/cache/syscache.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/cache/syscache.c,v 1.75 2002/04/09 20:35:54 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/cache/syscache.c,v 1.76 2002/04/11 20:00:06 tgl Exp $
  *
  * NOTES
  *	  These routines allow the parser/planner/executor to perform
@@ -93,22 +93,12 @@ struct cachedesc
 };
 
 static const struct cachedesc cacheinfo[] = {
-	{AggregateRelationName,		/* AGGNAME */
-		AggregateNameTypeIndex,
-		0,
-		2,
-		{
-			Anum_pg_aggregate_aggname,
-			Anum_pg_aggregate_aggbasetype,
-			0,
-			0
-	}},
-	{AggregateRelationName,		/* AGGOID */
-		AggregateOidIndex,
+	{AggregateRelationName,		/* AGGFNOID */
+		AggregateFnoidIndex,
 		0,
 		1,
 		{
-			ObjectIdAttributeNumber,
+			Anum_pg_aggregate_aggfnoid,
 			0,
 			0,
 			0