There is an alternate git history where I continued the Forth-ification of
words, including "SKIP?", but that was a bad idea: because that word was
written by flow control immediates, I stepped into quicksands where stability
became necessary in z80c.fs and I couldn't gracefully get out of it.

I'm stepping back and take this opportunity to replace the shoddy SKIP? algo
with a more straightforward (?br) implementation.

(br) and (?br) will always stay in boot code where it's easier manage a stable
ABI.

I can't get rid of "(fbr)" and "(bbr)" just yet, but soon...

Because icore.fs defines it in a "not-for-runtime" manner, dummy.fs
executing it had ill effects. We use a workaround instead.

Lifting ourselves by the bootstraps!

It's not needed anymore

Ain't that not self-bootstrapping enough to your taste? Whoa, I'm
getting dizzy...

Now we're seriously getting into real boostrapping territory...

This limitation was becoming annoying...

emul/forth/stage{0,1}.asm now only configure constants and append
binaries.

They do save space (nearly 100b), but they complicate our Forth-ification
process. We'll re-add them later.

This allows us to save a whole 500 bytes on the final binary size!

This change comes after I took a look at the hex dump and saw that one letter
constants in z80a.fs took a lot of space.

The removal of the "divide" routine highlighted the fact that
nativeWord wasn't properly stabilized.