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/include/utils/syscache.h

@@ -80,6 +80,8 @@ enum SysCacheIdentifier
 	RULERELNAME,
 	STATRELATTINH,
 	TABLESPACEOID,
+	TRFOID,
+	TRFTYPELANG,
 	TSCONFIGMAP,
 	TSCONFIGNAMENSP,
 	TSCONFIGOID,