It doesn't sound like much, but that backspace key is often useful...

This huge refactoring remove the Seek and Tell routine from blockdev
implementation requirements and change GetC and PutC's API so that they
take an address to read and write (through HL/DE) at each call.

The "PTR" approach in blockdev implementation was very redundant from
device to device and it made more sense to generalize. It's possible
that future device aren't "random access", but we'll be able to add more
device types later.

Another important change in this commit is that the "blockdev handle" is
now opaque. Previously, consumers of the API would happily call routines
directly from one of the 4 offsets. We can't do that any more. This
makes the API more solid for future improvements.

This change forced me to change a lot of things in fs, but overall,
things are now simpler. No more `FS_PTR`: the "device handle" now holds
the active pointer.

Lots, lots of changes, but it also feels a lot cleaner and solid.

This allows us to break through the 64K limit for includes CFS in zasm,
a limit we were dangerously close to breaking. In fact, this commit
makes us go over that limit. Right in time!

Let go of that "meta os" thing. it's not as meta as I made it sound
like. It's a kernel.

Will be much much easier to tests new core routines without having to
re-create their context first.

Also, extract parse.asm from core.asm

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.

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`

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.

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

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.

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.