This was becoming heavy to manage.

This is the first commit I do entirely in VE. It's a habit I'm planning
on taking as it helps a lot to find usability issues.

Use straight VARIABLE instead of Z80MEM+. Initially, I used this
system to allow z80a to be embedded in a system binary, but now
I don't think it's worth it. Compiled, z80a is 2.5k. Sure, it's a
sizeable amount of RAM, but I think that even with it in RAM, I'll
manage a bootstrap within my most constrained machine, the SMS with
8K.

And use them in all places they make sense.

Will be useful for assembling binaries for the TRS-80 which can't start
at addr 0.

This allows us to remove forth/z80a.fs. Another big step towards
self-hosting!

I don't remove z80a.fs yet because build process needs it, but that
will change shortly.