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/hstore_plpython/sql/hstore_plpython.sql

@@ -0,0 +1,103 @@
+CREATE EXTENSION plpython2u;
+CREATE EXTENSION hstore_plpython2u;
+
+
+-- test hstore -> python
+CREATE FUNCTION test1(val hstore) RETURNS int
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE hstore
+AS $$
+assert isinstance(val, dict)
+plpy.info(sorted(val.items()))
+return len(val)
+$$;
+
+SELECT test1('aa=>bb, cc=>NULL'::hstore);
+
+
+-- the same with the versioned language name
+CREATE FUNCTION test1n(val hstore) RETURNS int
+LANGUAGE plpython2u
+TRANSFORM FOR TYPE hstore
+AS $$
+assert isinstance(val, dict)
+plpy.info(sorted(val.items()))
+return len(val)
+$$;
+
+SELECT test1n('aa=>bb, cc=>NULL'::hstore);
+
+
+-- test hstore[] -> python
+CREATE FUNCTION test1arr(val hstore[]) RETURNS int
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE hstore
+AS $$
+plpy.info(repr(val))
+return len(val)
+$$;
+
+SELECT test1arr(array['aa=>bb, cc=>NULL'::hstore, 'dd=>ee']);
+
+
+-- test python -> hstore
+CREATE FUNCTION test2() RETURNS hstore
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE hstore
+AS $$
+val = {'a': 1, 'b': 'boo', 'c': None}
+return val
+$$;
+
+SELECT test2();
+
+
+-- test python -> hstore[]
+CREATE FUNCTION test2arr() RETURNS hstore[]
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE hstore
+AS $$
+val = [{'a': 1, 'b': 'boo', 'c': None}, {'d': 2}]
+return val
+$$;
+
+ SELECT test2arr();
+
+
+-- test as part of prepare/execute
+CREATE FUNCTION test3() RETURNS void
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE hstore
+AS $$
+rv = plpy.execute("SELECT 'aa=>bb, cc=>NULL'::hstore AS col1")
+plpy.info(repr(rv[0]["col1"]))
+
+val = {'a': 1, 'b': 'boo', 'c': None}
+plan = plpy.prepare("SELECT $1::text AS col1", ["hstore"])
+rv = plpy.execute(plan, [val])
+plpy.info(repr(rv[0]["col1"]))
+$$;
+
+SELECT test3();
+
+
+-- test trigger
+CREATE TABLE test1 (a int, b hstore);
+INSERT INTO test1 VALUES (1, 'aa=>bb, cc=>NULL');
+SELECT * FROM test1;
+
+CREATE FUNCTION test4() RETURNS trigger
+LANGUAGE plpythonu
+TRANSFORM FOR TYPE hstore
+AS $$
+plpy.info("Trigger row: {'a': %r, 'b': %r}" % (TD["new"]["a"], TD["new"]["b"]))
+if TD["new"]["a"] == 1:
+    TD["new"]["b"] = {'a': 1, 'b': 'boo', 'c': None}
+
+return "MODIFY"
+$$;
+
+CREATE TRIGGER test4 BEFORE UPDATE ON test1 FOR EACH ROW EXECUTE PROCEDURE test4();
+
+UPDATE test1 SET a = a;
+SELECT * FROM test1;