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.
