The dual scraptchpad thing doesn't work. Things become very
complicated when it's time to write that back to the file. We
overwrite our contents and end up with garbage.

This hard-binds ed to the filesystem (I liked the idea of working
only with blockdevs though...), but this is necessary for the
upcoming `w` command. We need some kind of way to tell the
destination to write to truncate itself.

This only has a meaning in the filesystem, but it's necessary to
let the file know that its registered file size has possibly
shrunk.

I thought of alternatives that would have allowed me to keep ed
blkdev-centered, but they were all too hackish to my own taste.

Hence, this new hard-bind on files.

During expression parsing, if a local label was parsed, it would
select the local registry and keep that selection, making
subsequent global labels register in the wrong place.

It was mostly wrong.

Starting to feel interactive...

That was an interesting bug. It didn't cause a problem in emulation, but
in an RC2014 on an SD card, an include that didn't end with two newlines
would cause an infinite loop.

Needed if we want to compile the kernel and zasm from within a SD card.
I didn't go straight for 32-bit because it was significantly more
complex and 24-bit give us 16M. Enough to go on for a while...

Weren't properly saved during local pass back and forth.

That's why the command seemed slow! It's much faster than I thought.

This allows us to write to an AT28 EEPROM from within collapse os.

A new app to stress test the SD card driver. Also, accompanying this
commit, changes solidifying the SD card driver so that stress tests
actually pass :)

Previous refacoring broke all seek/tell within fs. fs handles now lost
the responsibility to keep track of current position. It's blkdev's job.

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.

What used to be `tools/emul/user.h` was in fact specific to zasm, so I
moved it there.

To avoid name confusion, I renamed what used to be kernel.h and user.h
to kernel-bin.h and user-bin.h.

Using HL/DE was awkward and error-prone.

For now, top-level only