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...