It doesn't change anything featurewise, but this change is in
preparation for the addition of an eventual AT-XY.

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 that adev is directly in core, things are easier.

It's small enough to be worth it.

Also, fix PSP leak in font compilers.

The keyboard is a bit glitchy, but these are the same glitch as those
that were already present in Z80 Collapse OS. They still need fixing...

Addressed devices are not included in Collapse OS by default. Let's
just accept that upload only works with memory.

AGAIN, becomes a shortcut for "' RJMP AGAIN?,"

I decided to keep SKIP, .. TO, (renamed from AT,) around and limit
IF, .. THEN, to the simple BRNE case.

Will change to IF, .. THEN, but I need a way to easily reverse a
BR op. But from this commit, the translation of sms/kbd/ps2ctl.asm
is complete! perfect binary match!

Still making great progress on sms/kbd/ps2ctl. Still matching ref
binary.

next step: implement ST.

Getting good! still advancing on ps2ctl in sms/kbd and still
matching reference binary.

This will make it easier to fit BR* in there.

Still binary matching. Next step is branching support.

So far, the resulting binary matches.

This allows us to remove a lot of labels usage in boot code. This
commit has no effect on forth.bin.

and renamed it "Core words". Also, reworded the presentation.

More portable