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

I wrote:
> What I am mainly wondering about at this point is whether Asserts
> are good enough or we should use actual test-and-elog checks for
> these things.

Hearing no comments on that, I decided that a good policy would be
to use Asserts in the paths dealing with small chunks but test-and-elog
in the paths dealing with large chunks. We already had test-and-elog
sanity checks in the latter paths, at least in aset.c, which the new
checks can reasonably be combined with. It's unlikely that the
large-chunk case is at all performance-critical, too. But adding
runtime checks in the small-chunk paths would probably risk losing
some performance in production builds, and I'm not currently prepared
to argue that the problem is big enough to justify that.

Hence v2 attached, which cleans things up a tad in aset.c and then
extends similar policy to generation.c and slab.c. Of note is
that slab.c was doing things like this:

SlabContext *slab = castNode(SlabContext, context);

Assert(slab);

which has about the same effect as what I'm proposing with
AllocSetIsValid, but (a) it's randomly different from the
other allocators, and (b) it's probably a hair less efficient,
because I doubt the compiler can optimize away castNode's
special handling of NULL. So I made these bits follow the
style of aset.c.

regards, tom lane