Proposed patch for sequence-renaming problems

Attached is a fully-worked-out patch to make SERIAL column default
expressions refer to the target sequence with a "regclass" literal
instead of a "text" literal. Since the regclass literal is actually
just an OID, it is impervious to renamings and schema changes of
the target sequence. This fixes the long-standing hazard of renaming
a serial column's sequence, as well as the recently added hazard of
renaming the schema the sequence is in; and it lets us get rid of a
very klugy solution in ALTER TABLE SET SCHEMA.

I've arranged for stored regclass literals to create dependencies on
the referenced relation, which provides useful improvements even for
handwritten defaults: given

create sequence myseq;
create table foo (f1 int default nextval('myseq'::regclass));

the system will not allow myseq to be dropped while the default
expression remains. (This also ensures that pg_dump will emit the
sequence before the table.)

The patch also fixes a couple of places where code was still looking
at the deprecated pg_attrdef.adsrc column, instead of reverse-compiling
pg_attrdef.adbin. This ensures that psql's \d command shows the
up-to-date form of a column default. (That should have happened quite
some time ago; not sure why it was overlooked.)

I propose applying this to fix the open issue that ALTER SCHEMA RENAME
breaks serial columns. Comments, objections?

regards, tom lane