Cross-compiled core

This units contains core Collapse OS that are cross-compiled.
During building, these 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. It cannot compile a word from higher layers. Immediates are
   fine.

                                                        (cont.)