Core words

This section contains arch-independent core words of Collapse
OS. Those words are written in a way that make them entirely
cross-compilable (see B260). When building Collapse OS, these
words come right after the boot binary (B280).

Because this unit is designed to be cross-compiled, things are
a little weird. It is compiling in the context of a full
Forth interpreter with all bells and whistles (and z80
assembler), but it has to obey strict rules:

1. Although it cannot compile a word that isn't defined yet,
   it can still execute an immediate from the host system.

                                                        (cont.)
