ref #82

VE works! well, at this point, we can say that Collapse OS runs on
PC/AT. A little cleanup and docs writing and we're done!

It helps a bit with staging times.

On real hardware, there aren't always 2 heads...

instead of hardcoded drive 0.

That INT 13H logic was a bit tricky to implement...

read-only for now.

Also, fix (roll) which wasn't properly implemented.

It isn't used by low level boot code anymore, so it can go there.

Was previously referred to by name+find, which was wasteful.

This commit adds ?DUP IF guards to MOVE and MOVE- that make them exit
when u is 0. Without these guards a DO loop was executed 65535 times
instead of 0.

In particular, this fixes a crash when "105 LOAD I " is executed
immediately after boot. Block editor word "I" passes IBUF length (0)
to MOVE- and MOVE in this case, causing a crash.

Forth intepreter now works! There's still DOES> and overflow checks
to implement, but otherwise, we have it!

Was required for the many "256 /MOD" in the code.

Not working, but we do get that sweet "CollapseOS".

I always keep it on myself. I can imagine it's especially useful to
newcomers.

I've spent way too much time debugging bugs caused by this...

*sigh*, why can't I learn?

(well, not PC@ and PC!, but I'm not even sure what would be the role
of these in a PC/AT. they're only used in drivers on the z80 front,
so they will not be immediately needed. todo... )

Now comes the fitting part.

almost there!

This port is taking shape!

8086 asm is a lot terser than z80... Those indirect memory operations
are very handy.

We now have a stable ABI that is pretty cleared up...

The only calls still using the stable ABI indirection are those
made before the next routine is created.

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

Instead of having wordref point to core word routines, I made them
into word 4 word types. It liberates space into the stable ABI and
should make porting to other arches easier.

I'm also thinking of combining word type with the namelen field
for precious bytes saving, but not now...

I broke them when I removed call indirection layers without also
removing usage of BCALL.

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.

Only 2>R in actually needed in the RSP family (for LOOP). Reducing
the scope of the stable ABI makes porting to other arches easier.