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.

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.

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...

My mind isn't so good at decimal -> hex translations. It helps to have
this...

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.

Now that we don't sed out the comments any more, we busted the buffer.

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

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!

For now, top-level only

Up until now, invalid source input had undefined behavior. We're now
beginning to define that behavior so that zasm can be a bit more usable.

To run a parseExpr on first pass would always return a false success
with dummy value because symbols are configured to always succeed on
first pass. This would make expressions like ".fill 0x38-$" so bad
things to labels because "0x38-$" wouldn't return the same thing on
first and second pass.

Revert to parsing literals and symbols after having scanned for
expressions and add a special case specifically for char literals (which
is why we scanned for literals and symbols first in the first place).

Allows for the ".fill x-$" pattern.

It was a bad idea to remove it. Now that I'm introducing the concept of
a per-app glue file, it becomes much easier to build emulated zasm as a
userspace app.

Made things complicated for nothing.

Make symRegister's logic pointer-based so we can break through the 0x100
limit.