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.

Replaces individual command parsing. Quite a big and complex chunk of
code, but makes each command much simpler.

Easier to use this way...

They serve a different purpose. The goal of the ACIA buffer is to ensure
that we don't miss an input. The goal of the shell buffer is to wait
until the user presses return.

The ACIA buffer has been moved to shell and replaced with a circular
buffer, a more appropriate data structure for this kind of purpose.

Also, introduce `aciaGetC`.

Also, fix seek reversed endianness.

This introduces `core.asm` which includes routines used by other parts.

This change introduces a command name matching algo.

Also, come up with a way to make parts play well together memory-wise.