The idea is to consider assemblers as "runtime" apps instead of
placing them in the "bootstrap" section of the blocks. These apps
will be used for much more than bootstrapping.

Moved its documentation to doc/asm.txt and made its code blocks
more compact.

The previous approach of maintaining R> and W> pointers was
conceptually simple, but made INT handler code actually quite
complex.

Now, we maintain indexes instead. It's much easier to perform
bounds checks and to compare for equality, something we have to
do quick in the INT handler.

Initially, I used the same letters as those used in the z80 ref
docs, but it makes the different assemblers harder to use than they
should. Having consistent "argtype" rules across assemblers should
help.

This was becoming heavy to manage.

This is the first commit I do entirely in VE. It's a habit I'm planning
on taking as it helps a lot to find usability issues.

I don't remove z80a.fs yet because build process needs it, but that
will change shortly.