Needed by #30.

It needs bash after all. On OpenBSD, /usr/sh works, but not on Ubuntu.

* Fix for tools/zasm.sh being dependent on readlink -f (an issue on macOS, preventing builds)

* Wrap zasm.sh shebang in /usr/bin/env ; remove comment about BSDs

I've tested RAM usage when self-assembling and there weren't as high
as I thought. zasm's defaults now use less than 0x1800 bytes of RAM,
making it possible, theoretically for now, for a Sega Master System
to assemble Collapse OS from within itself.

scas, it's not needed any more.

This makes zasm suitable to run from ROM.

This will allow interesting override scenarios, adding flexibility.

Instead of strings of variable length driving the iteration of the
registry, we do so through records that keep track of lengths and
counts.

*sob*...

I'm about to split the global registry in two (labels and consts)
and the previous state of registry selection made things murky.
Now it's much better.

Also, remove zasm/test7 because it changes too much all time time
(whenever zasm changes) and isn't precise enough. Too much noise,
not worth it.

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.

I'm about to break compatibility with scas. Before I do that, I
need to adjusts tests. Instead of running scas to compare results,
we commit expected result as binaries directly in the repo.

Also, make tests run in a *BSD environment

Also, rename "download.py" to "memdump.py".

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.

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...