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.

When I zasm-ified the shell a couple of dozens of commits ago, I
inverted push and pops...

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.

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

Made things complicated for nothing.

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

But we're still at a certain distance from assembling it with zasm:
we're busting the 0x100 symbol limit.

Previously, we would mess up literals like '-'.

Also, make zasm assemble zasm/util.asm again!