Big bootstrapping change which greatly reduces the role of the modern
operating system in the bootstrapping process.

More reliable.

Less noisy this way

This allows us to remove forth/z80a.fs. Another big step towards
self-hosting!

To update LATEST offset more easily. Will be useful for recipes.

Make LATEST configurable through a value in the binary instead of through
a ZASM constant. This enables the simple concatenation of compiled dicts and
simplifies the build process.

Now we're having a real nice and tidy forth.asm...

They do save space (nearly 100b), but they complicate our Forth-ification
process. We'll re-add them later.

This allows us to save a whole 500 bytes on the final binary size!

This change comes after I took a look at the hex dump and saw that one letter
constants in z80a.fs took a lot of space.

This requires us to significantly adjust our build process, which
now has 3 stages.

The goal being to add a new native code dict staging phase.

This should help with upcoming challenges in bootstrapping z80 code
compiled through Forth.

My approach with RS was slightly wrong: RS' TOP was always containing current
IP. It worked, but it was problematic when came the time to introduce
RS-modifying words: it's impossible to modify RS in a word without immediately
messing your flow.

Therefore, what used to be RS' TOS has to be a variable that isn't changed
midway by RS-modifying words. I guess that's why RS is called *return* stack...

Big one.

This allows us to write higher order words directly in Forth, which is much
more convenient than writing post-immediate (see "NOT" structure in diff if
you want to see what I mean) structures in ASM.

These structures can then be written to ROM (rather than loaded in RAM for
definitions loaded at run-time).

That's quite a bit of tooling that was added, 2 compilations stages, but I
think it's well worth it.