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.

It was useful when we still had the relinker, but now it has no use.
I was waiting a bit to see if the distinction would be useful again,
but it seems like it won't.

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.