Re: Reducing the chunk header sizes on all memory context types

Here's a v2 incorporating discussed changes.

In reordering enum MemoryContextMethodID, I arranged to avoid using
000 and 111 as valid IDs, since those bit patterns will appear in
zeroed and wipe_mem'd memory respectively. Those should probably be
more-or-less-permanent exclusions, so I added comments about it.

I also avoided using 001: based on my work with converting guc.c to use
palloc [1], it seems that pfree'ing a malloc-provided pointer is likely
to see 001 a lot, at least on 64-bit glibc platforms. I've not stuck
my nose into the glibc sources to see how consistent that might be,
but it definitely recurred several times while I was chasing down
places needing adjustment in that patch.

I'm not sure if there are any platform-dependent reasons to avoid
other bit-patterns, but we do still have a little bit of flexibility
left here if anyone has candidates.

regards, tom lane

[1] https://www.postgresql.org/message-id/2982579.1662416866%40sss.pgh.pa.us