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/include/nodes/primnodes.h

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: primnodes.h,v 1.60 2002/03/26 19:16:53 tgl Exp $
+ * $Id: primnodes.h,v 1.61 2002/04/11 20:00:15 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -322,13 +322,9 @@ typedef struct Iter
 typedef struct Aggref
 {
 	NodeTag		type;
-	char	   *aggname;		/* name of the aggregate */
-	Oid			basetype;		/* base type Oid of the aggregate (ie,
-								 * input type) */
-	Oid			aggtype;		/* type Oid of final result of the
-								 * aggregate */
-	Node	   *target;			/* attribute or expression we are
-								 * aggregating on */
+	Oid			aggfnoid;		/* pg_proc Oid of the aggregate */
+	Oid			aggtype;		/* type Oid of result of the aggregate */
+	Node	   *target;			/* expression we are aggregating on */
 	bool		aggstar;		/* TRUE if argument was really '*' */
 	bool		aggdistinct;	/* TRUE if it's agg(DISTINCT ...) */
 	int			aggno;			/* workspace for executor (see nodeAgg.c) */