Inner core

This unit represents core definitions that happen right after
native definitions. Before core.fs.

Unlike core.fs and its followers, this unit isn't self-
sustained. Like native defs it uses the machinery of a full
Forth interpreter, notably for flow structures.

Because of that, it has to obey specific rules:

1. It cannot compile a word from higher layers. Using
   immediates is fine though.


                                                        (cont.)