WIP -- renaming implicit sequences

Hi,

Here is an unfinished patch to implement something which appears on
the TODO list under ALTER: automatic renaming of sequences created
with serial when the table and column names change.  I've often wanted
this feature and it seemed like a good starter project.  I'd be
grateful for any feedback and advice on how I could get it into
acceptable shape.  Example:

hack=# create table foo (id serial primary key);
NOTICE:  CREATE TABLE will create implicit sequence "foo_id_seq" for
serial column "foo.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index
"foo_pkey" for table "foo"
CREATE TABLE
hack=# alter table foo rename to bar;
NOTICE:  ALTER TABLE will rename implicit sequence "foo_id_seq" to "bar_id_seq"
ALTER TABLE
hack=# alter table bar rename id to snacks;
NOTICE:  ALTER TABLE will rename implicit sequence "bar_id_seq" to
"bar_snacks_seq"
ALTER TABLE

Sequences are considered to be renameable if they are owned by the
table, and have a name conforming to the name pattern used by CREATE
TABLE (table_column_seq with optional trailing numbers).  If you've
manually renamed a SEQUENCE so that it doesn't conform, it won't touch
it.  If you've created a SEQUENCE and declared it to be OWNED BY the
table, then it will be renamed only if it happens to conform.

I'm not sure what to do about permissions.  I guess it should silently
skip renaming sequences if the user doesn't have appropriate
privileges.

Useful?  Why would anyone not want this behaviour?  Have I used
inappropriate locking levels?  What should I read to understand the
rules of locking?  Have I failed to handle errors? Have I made memory
ownership mistakes?

Thanks!
Thomas Munro