Add new CREATE CONVERSION/DROP CONVERSION command.

This is the first cut toward CREATE CONVERSION/DROP CONVERSION implementaion.
The commands can now add/remove tuples to the new pg_conversion system
catalog, but that's all. Still need work to make them actually working.
Documentations, regression tests also need work.

src/include/commands/conversioncmds.h

@@ -0,0 +1,23 @@
+/*-------------------------------------------------------------------------
+ *
+ * conversioncmds.h
+ *	  prototypes for conversioncmds.c.
+ *
+ *
+ * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $Id: conversioncmds.h,v 1.1 2002/07/11 07:39:27 ishii Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef CONVERSIONCMDS_H
+#define CONVERSIONCMDS_H
+
+#include "nodes/parsenodes.h"
+
+extern void CreateConversionCommand(CreateConversionStmt *parsetree);
+extern void DropConversionCommand(List *conversion_name);
+
+#endif  /* CONVERSIONCMDS_H */