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.

A python script generates all possibilities for all supported
instructions and compare zasm output with scas. After having fixed a
couple of bugs, all tests pass!

This facilitates debugging. To know the value of `A` at any point,
you can do `out (0), a`. The number of bytes in the output will be the
value of `A`.

Looping is easier

Previously, we would go the other way around: match all available
argspecs to raw argument strings. It's much better to go the other way
around because we can "reduce" each argument much sooner. That will be
important when we start to support constants and expressions.

These routines had hardcoded references to ACIA and didn't belong to the
core.

We now properly match arg-less operations.

Single opcodes that yield constants. "echo ret | zasm" yields 0xc9.

zasm is going to need to call to kernel code...

For now, only a dummy app, but it's emulated properly with libz80.
Exciting times!

Also, remove `parts/platforms`. It makes no sense. Only recipes make
sense.

The approach used for shell extra commands also works for blockdev. Much
simpler and tighter this way.

This also introduces the "seek" shell command.

Also, add dummy seek reoutine.

going to use `seek` for block devices

Also, add the ability for parts to define extra shell commands.

This will facilitate the inclusion of extra commands other parts might
want to define.