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/nodes/parsenodes.h

@@ -1265,6 +1265,7 @@ typedef enum ObjectType
 	OBJECT_TABCONSTRAINT,
 	OBJECT_TABLE,
 	OBJECT_TABLESPACE,
+	OBJECT_TRANSFORM,
 	OBJECT_TRIGGER,
 	OBJECT_TSCONFIGURATION,
 	OBJECT_TSDICTIONARY,
@@ -2789,6 +2790,20 @@ typedef struct CreateCastStmt
 	bool		inout;
 } CreateCastStmt;
 
+/* ----------------------
+ *	CREATE TRANSFORM Statement
+ * ----------------------
+ */
+typedef struct CreateTransformStmt
+{
+	NodeTag		type;
+	bool		replace;
+	TypeName   *type_name;
+	char	   *lang;
+	FuncWithArgs *fromsql;
+	FuncWithArgs *tosql;
+} CreateTransformStmt;
+
 /* ----------------------
  *		PREPARE Statement
  * ----------------------