Add support for invoking parser callback hooks via SPI and in cached plans.

As proof of concept, modify plpgsql to use the hooks.  plpgsql is still
inserting $n symbols textually, but the "back end" of the parsing process now
goes through the ParamRef hook instead of using a fixed parameter-type array,
and then execution only fetches actually-referenced parameters, using a hook
added to ParamListInfo.

Although there's a lot left to be done in plpgsql, this already cures the
"if (TG_OP = 'INSERT' and NEW.foo ...)"  problem, as illustrated by the
changed regression test.

src/include/executor/spi_priv.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/spi_priv.h,v 1.32 2009/01/01 17:23:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/executor/spi_priv.h,v 1.33 2009/11/04 22:26:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -68,6 +68,8 @@ typedef struct _SPI_plan
 	int			cursor_options; /* Cursor options used for planning */
 	int			nargs;			/* number of plan arguments */
 	Oid		   *argtypes;		/* Argument types (NULL if nargs is 0) */
+	ParserSetupHook parserSetup;	/* alternative parameter spec method */
+	void	   *parserSetupArg;	
 } _SPI_plan;
 
 #endif   /* SPI_PRIV_H */