Refactor "ALTER some-obj SET SCHEMA" implementation

Instead of having each object type implement the catalog munging
independently, centralize knowledge about how to do it and expand the
existing table in objectaddress.c with enough data about each object
type to support this operation.

Author: KaiGai Kohei
Tweaks by me
Reviewed by Robert Haas

src/include/nodes/parsenodes.h

@@ -2168,7 +2168,6 @@ typedef struct AlterObjectSchemaStmt
 	RangeVar   *relation;		/* in case it's a table */
 	List	   *object;			/* in case it's some other object */
 	List	   *objarg;			/* argument types, if applicable */
-	char	   *addname;		/* additional name if needed */
 	char	   *newschema;		/* the new schema */
 	bool		missing_ok;		/* skip error if missing? */
 } AlterObjectSchemaStmt;