Making Bitmapsets be valid Nodes

Per the discussion at [1], it seems like it'd be a good idea to make
Bitmapsets into full-fledged, tagged Nodes, so that we could do things
like print or copy lists of them without special-case logic. The
extra space for the NodeTag is basically free due to alignment
considerations, at least on 64-bit hardware.

Attached is a cleaned-up version of Amit's patch v24-0003 at [2].
I fixed the problems with not always tagging Bitmapsets, and changed
the outfuncs/readfuncs logic so that Bitmapsets still print exactly
as they did before (thus, this doesn't require a catversion bump).

As proof of concept, I removed the read_write_ignore labels from
RelOptInfo's unique_for_rels and non_unique_for_rels fields, and
got nice-looking debug printout:

:unique_for_rels ((b 1))
:non_unique_for_rels <>

I also removed some special-case code from indxpath.c because
list_member() can do the same thing now. (There might be other
places that can be simplified; I didn't look very hard.)

It'd be possible to make Bitmapset fields be (mostly) not special cases
in the copy/equal/out/read support. But I chose to leave that alone,
because it'd add a little runtime overhead for indirecting through the
generic support functions while not really saving any code space.

Barring objections, I'd like to go ahead and push this.

regards, tom lane

[1] https://www.postgresql.org/message-id/94353655-c177-1f55-7afb-b2090de33341%40enterprisedb.com
[2] https://www.postgresql.org/message-id/CA%2BHiwqEYCLRZ2Boq_uK0pjLn_9b8XL-LmwKj7HN5kJOivUkYLg%40mail.gmail.com