It was previously skipped on a local label.

That is, labels with code following right afterwards.

With local labels, these two will have to be decoupled.

Facilitates debugging

Gives clearer output on errors. Easier debugging...

`xor c` didn't do what I thought it did at the time. Newbie mistake,
there are probably a lot like this scattered around...

Also, add warning on attempts to write to ROM.

scas didn't warn about this, but it seems that `jr` calls were out of
range. Seek and Tell was broken. Replacing `jr` with `jp` does the
trick.

With the help of the newly-introduced cfspack tool, we can mount a
filesystem in our emulated shell and play around. Read-only for now.
Unpacking incoming.

Also, confirm in tests that we can refer to other symbols in `.equ`.

Also, add a port for rewinding stdin.

This will soon allow use to seek and tell on input, which is necessary
for a second pass which is necessary for forward symbol references.

This require making `blkSel` a bit more flexible. Rather than having one
global selected blkdev, each app can select its own, in its own memory
space.

I realize that libz80 is GPL and that this project is MIT. That's not
much of a problem as long as libz80 isn't a big part of the project, but
I'm planning on building more around libz80 so that zasm can replace
scas as an assembler even on modern machines (through an emulated
layer) so that this project can become entirely self-bootstrapped.

If I'm going to do that, I might as well be kosher license-wise. I
don't care much about whether the project is GPL or MIT...

Also, adjust SD card recipe. Straightforward initialization and read!

Makes for a more straightforward test.

Incomplete, but what it does *does* work. I could handshake CMD0+CMD8 on
a MicroSD HC.