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.

Modest first step