Rules on table partitions

Currently we allow rules to be defined on table partitions, but these
rules only fire when the partition is accessed directly, not when it
is accessed via the parent:

CREATE TABLE t1(a int, b int) PARTITION BY RANGE(a);
CREATE TABLE t1_p PARTITION OF t1 FOR VALUES FROM (1) TO (10);
INSERT INTO t1 VALUES (1,101), (2,201);

CREATE TABLE t1_p_log(a int, b int, d date);
CREATE RULE t1_p_log_rule AS ON UPDATE TO t1_p
DO ALSO INSERT INTO t1_p_log VALUES(old.a, old.b, now());

UPDATE t1 SET b=b+1 WHERE a=1;
UPDATE t1_p SET b=b+1 WHERE a=2;

SELECT * FROM t1_p_log;

a | b | d
---+-----+------------
2 | 201 | 2017-06-19
(1 row)

I'd regard that as a bug, especially since this kind of thing would
have worked with old-style user-defined partitioning. Perhaps we
should explicitly forbid this for now -- i.e., raise a "not supported"
error when attempting to add a rule to a partition, or attach a table
with rules to a partitioned table.

Personally, I wouldn't regard adding proper support for rules on
partitions as a high priority, so I'd be OK with it remaining
unsupported unless someone cares enough to implement it, but that
seems preferable to leaving it partially working in this way.

Also, as things stand, it is possible to do the following:

CREATE TABLE t2(a int, b int) PARTITION BY RANGE(a);
CREATE TABLE t2_p PARTITION OF t2 FOR VALUES FROM (1) TO (10);
CREATE RULE "_RETURN" AS ON SELECT TO t2_p DO INSTEAD SELECT * FROM t2;

which results in the partition becoming a view that selects from the
parent, which surely ought to be forbidden.

Regards,
Dean