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/expected/ltree_plpython.out

@@ -0,0 +1,45 @@
+CREATE EXTENSION plpython2u;
+CREATE EXTENSION ltree_plpython2u;
+CREATE FUNCTION test1(val ltree) RETURNS int
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE ltree
+AS $$
+plpy.info(repr(val))
+return len(val)
+$$;
+SELECT test1('aa.bb.cc'::ltree);
+INFO:  ['aa', 'bb', 'cc']
+CONTEXT:  PL/Python function "test1"
+ test1 
+-------
+     3
+(1 row)
+
+CREATE FUNCTION test1n(val ltree) RETURNS int
+LANGUAGE plpython2u
+TRANSFORM FOR TYPE ltree
+AS $$
+plpy.info(repr(val))
+return len(val)
+$$;
+SELECT test1n('aa.bb.cc'::ltree);
+INFO:  ['aa', 'bb', 'cc']
+CONTEXT:  PL/Python function "test1n"
+ test1n 
+--------
+      3
+(1 row)
+
+CREATE FUNCTION test2() RETURNS ltree
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE ltree
+AS $$
+return ['foo', 'bar', 'baz']
+$$;
+-- plpython to ltree is not yet implemented, so this will fail,
+-- because it will try to parse the Python list as an ltree input
+-- string.
+SELECT test2();
+ERROR:  syntax error at position 0
+CONTEXT:  while creating return value
+PL/Python function "test2"