Makes for a more straightforward test.

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

Without delays between read/writes, I often add lockups.

It always reads in the scratchpad with the same max size. No need for
DE-dancing anymore.

De-couple "success" from "wrote something to (DE)" so that directive
parsing can be considered successful without writing to (DE).

It will make tokenization cleaner and it also makes the instruction
table significantly more compact.

instr.asm is for single instruction assembly.

* Build emulated zasm statically
* Improve comments in zasm.asm
* Fix build
* Use unsetZ from core

Also, clarify the role of recipes.

Very coarse, but I got them to work.

This way, we can interact with the mounted FS even when we change the
selected blkdev.

This will facilitate "copying" blk selection in FS.

This allows more than one file to be allocated. `fls` properly lists
multiple files.

Very incomplete, but if you play your cards right, you can, with the
shell:

1. Create a new CFS
2. Mount it
3. Allocate a file with an arbitrary name
4. Have it listed with `fls`

It wasn't a good idea to complicate all blockdev impls with complicated
seeks. Let's just stay in absolute mode for now.

Also, extract commands into blockdev_cmds to avoid making blockdev
dependent on stdio.

Allows seeking forward, backwards, to the beginning, to the end.

No consumer yet, but it's coming...

Calling on blkGetC repeatedly was somehow very unstable and often
failed. I didn't pinpoint exactly why, but keeping the call addr around
and calling that instead seems like a better idea anyway.

The opposite of "load"

Instead of waiting, GetC always return immediately, with Z indicating if
something was fetched. The "wait" loop is implemented by the called (and
in the new `blkGetCW`).

This simplifies out-of-bounds verifications for storage blockdevs.

Let's not create complex rules for exception cases.