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.

I'm about to change that structure and I don't want fs to be messed up
by this. I'm proceeding step by step...

This makes for a much more usable zasm linux binary that doesn't have to
be rebuilt every time apps or kernel change.

... instead of embedding it in the binary itself. Additionally, add a
"zasm.sh" wrapper to faciliate zasm calls on a linux machine.

Wriiiiiite!

The pgm module implements a shell hook so that when an unknown command
is typed, we look into the mounted filesystem and look for a file with
the same name as the command. If we find one, we load it in memory and
run it.

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

This will soon allow use to seek and tell on input, which is necessary
for a second pass which is necessary for forward symbol references.

This require making `blkSel` a bit more flexible. Rather than having one
global selected blkdev, each app can select its own, in its own memory
space.

Also, adjust SD card recipe. Straightforward initialization and read!

Makes for a more straightforward test.