ref #97

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...

To update LATEST offset more easily. Will be useful for recipes.

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

A bug in rdWS made zasm error out when omiting its 3rd argument.

fixes #90

I had forgotten about that ordering thing: depending on the platform
cfspack doesn't read files in a directory in the same order.

This allows us to get rid of the zasm.sh wrapper.