This is in preparation of using BC instead of IY for IP. I've
verified all PUSH* usage and none was problematic.

Initially, I used the same letters as those used in the z80 ref
docs, but it makes the different assemblers harder to use than they
should. Having consistent "argtype" rules across assemblers should
help.

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.

I have a working offset-by-0x100 stage0 now.

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