The SD card driver is a bit too slow to be bearable. I'll write
_crc16 in z80 and see how it goes.

Less noisy this way

And fix broken negative display in ".".

Why did I ever think that using CM was a good idea? any comparison
with numbers more than 0x8000 apart would be wrong...

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