This was becoming heavy to manage.

Instead of having wordref point to core word routines, I made them
into word 4 word types. It liberates space into the stable ABI and
should make porting to other arches easier.

I'm also thinking of combining word type with the namelen field
for precious bytes saving, but not now...

This is the first commit I do entirely in VE. It's a habit I'm planning
on taking as it helps a lot to find usability issues.

I'm not sure why I chose null-terminated initially. Probably because
the z80asm version had null-terminated strings.

Length-prefixes strings are the traditional form of strings in Forth
and it's a bit easier to work with them with traditional forth words
when they're under this form.

and renamed it "Core words". Also, reworded the presentation.

This leaves space for xcomp-core which is growing.