I'm planning a string reform and it's standing in the way.

Now that everything is cross-compiled, no need to XPACK. If we ever
need it again, we know where to find it.

Now that the boot binary is fully cross-compiled, there's no use for
the linker anymore. Theoretically, it could still be useful, but I
can't think of a real use case.

Let's take it out of the picture. If it's ever needed again, I'll
know where to find it.

I'm planning on going back to 8-bit branching. 16-bit br cells incur
a non-negligible penalty and, while at first 64 words (128 bytes
forward or backward) seemed a bit limiting, I now don't see why one
would ever construct such a big branch. It would be un-forthy.

Also, I looked at using BC instead of IY to hold IP and the transition
would be a lot easier with 8-bit branching.

In this commit, all I do is add overflow checks in IF. The mechanic
below doesn't change. I'll give myself some time to think it over so
that I avoid yet another back and forth.

Prebiously, when encountering an error during a : ; definition from
input buffer, because the input buffer wasn't flushed, we would continue
interpreting and quit the whole program when encountering ;.

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...

It's small enough to be worth it.

Also, fix PSP leak in font compilers.

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.

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.