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/utils/cache/syscache.c

@@ -56,6 +56,7 @@
 #include "catalog/pg_shseclabel.h"
 #include "catalog/pg_statistic.h"
 #include "catalog/pg_tablespace.h"
+#include "catalog/pg_transform.h"
 #include "catalog/pg_ts_config.h"
 #include "catalog/pg_ts_config_map.h"
 #include "catalog/pg_ts_dict.h"
@@ -653,6 +654,28 @@ static const struct cachedesc cacheinfo[] = {
 		},
 		4
 	},
+	{TransformRelationId,		/* TRFOID */
+	 TransformOidIndexId,
+	 1,
+	 {
+		 ObjectIdAttributeNumber,
+		 0,
+		 0,
+		 0,
+	 },
+	 16
+	},
+	{TransformRelationId,		/* TRFTYPELANG */
+	 TransformTypeLangIndexId,
+	 2,
+	 {
+		 Anum_pg_transform_trftype,
+		 Anum_pg_transform_trflang,
+		 0,
+		 0,
+	 },
+	 16
+	},
 	{TSConfigMapRelationId,		/* TSCONFIGMAP */
 		TSConfigMapIndexId,
 		3,