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/expected/hstore_plpython.out

@@ -0,0 +1,132 @@
+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);
+INFO:  [('aa', 'bb'), ('cc', None)]
+CONTEXT:  PL/Python function "test1"
+ test1 
+-------
+     2
+(1 row)
+
+-- 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);
+INFO:  [('aa', 'bb'), ('cc', None)]
+CONTEXT:  PL/Python function "test1n"
+ test1n 
+--------
+      2
+(1 row)
+
+-- 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']);
+INFO:  [{'aa': 'bb', 'cc': None}, {'dd': 'ee'}]
+CONTEXT:  PL/Python function "test1arr"
+ test1arr 
+----------
+        2
+(1 row)
+
+-- 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();
+              test2              
+---------------------------------
+ "a"=>"1", "b"=>"boo", "c"=>NULL
+(1 row)
+
+-- 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();
+                           test2arr                           
+--------------------------------------------------------------
+ {"\"a\"=>\"1\", \"b\"=>\"boo\", \"c\"=>NULL","\"d\"=>\"2\""}
+(1 row)
+
+-- 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();
+INFO:  {'aa': 'bb', 'cc': None}
+CONTEXT:  PL/Python function "test3"
+INFO:  '"a"=>"1", "b"=>"boo", "c"=>NULL'
+CONTEXT:  PL/Python function "test3"
+ test3 
+-------
+ 
+(1 row)
+
+-- test trigger
+CREATE TABLE test1 (a int, b hstore);
+INSERT INTO test1 VALUES (1, 'aa=>bb, cc=>NULL');
+SELECT * FROM test1;
+ a |           b            
+---+------------------------
+ 1 | "aa"=>"bb", "cc"=>NULL
+(1 row)
+
+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;
+INFO:  Trigger row: {'a': 1, 'b': {'aa': 'bb', 'cc': None}}
+CONTEXT:  PL/Python function "test4"
+SELECT * FROM test1;
+ a |                b                
+---+---------------------------------
+ 1 | "a"=>"1", "b"=>"boo", "c"=>NULL
+(1 row)
+