Add transforms feature

This provides a mechanism for specifying conversions between SQL data
types and procedural languages.  As examples, there are transforms
for hstore and ltree for PL/Perl and PL/Python.

reviews by Pavel Stěhule and Andres Freund

src/backend/catalog/pg_aggregate.c

@@ -545,6 +545,7 @@ AggregateCreate(const char *aggName,
 							 parameterModes,	/* parameterModes */
 							 parameterNames,	/* parameterNames */
 							 parameterDefaults, /* parameterDefaults */
+							 PointerGetDatum(NULL),	/* trftypes */
 							 PointerGetDatum(NULL),		/* proconfig */
 							 1, /* procost */
 							 0);	/* prorows */