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

contrib/ltree_plpython/ltree_plpython3u--1.0.sql

@@ -0,0 +1,12 @@
+-- make sure the prerequisite libraries are loaded
+DO '1' LANGUAGE plpython3u;
+SELECT NULL::ltree;
+
+
+CREATE FUNCTION ltree_to_plpython3(val internal) RETURNS internal
+LANGUAGE C STRICT IMMUTABLE
+AS 'MODULE_PATHNAME', 'ltree_to_plpython';
+
+CREATE TRANSFORM FOR ltree LANGUAGE plpython3u (
+    FROM SQL WITH FUNCTION ltree_to_plpython3(internal)
+);