with uflw checks not being in next anymore, we can save ourselves
a forward jump.

For the IY->BC move, of course.

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.

The usage of numerotated labels for this was becoming severly
confusing.

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 EXX instead of the stack for HL protection and remove all
spurious uses of chkPS,

I wanted to inline chkPS in next because of its "tight loop" status,
but for reasons I don't understand, doing so breaks Collapse OS.
Later...

Now I struggle to remember why I ever did things they way I did.
There must have been a reason...