Making type Datum be 8 bytes everywhere

In a discussion on Discord (in the PG #core-hacking channel,
which unfortunately is inaccessible to non-members), Andres
and Robert complained about the development/maintenance costs
of continuing to support 32-bit platforms. Here is a modest
proposal to reduce those costs without going so far as to
entirely desupport such platforms: let's require them to use
8-byte Datums even though that's probably not a native data
type for them. That lets us get rid of logic to support the
!USE_FLOAT8_BYVAL case, and allows a few other simplifications.

The attached patch switches to 8-byte Datums everywhere, but
doesn't make any effort to remove the now-dead code. I made
it just as a proof-of-concept that this can work. It compiled
cleanly and passed check-world for me on a 32-bit FreeBSD
image.

I've not looked into the performance consequences. We probably
should at least try to measure that, though I'm not sure what
our threshold of pain would be for deciding not to do this.

Thoughts?

regards, tom lane