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 allows us to get rid of the (parse*) indirection.

It's now a simple flag updated by the C< implementation. No more
routine pointer.

The goal is to avoid tons of "ok" during a LOAD operation.

This is done by adding a "C<* override" layer that is reset to 0 on
ABORT.

The problem was that when ABORT happened during a LOAD, we had a
resetted RSP and started from a clean INTERPRET, but LOAD didn't
have the opportunity to restore C<*, which caused it to continue
interpreting from the faulty BLK.

With a C<* override, we don't need to *restore* C<*, we just need to
clear the override.

This is more than cosmetic, it's also highly usable. The presence
or absence of the "ok" message allows us to know whether the command
aborted. Previously, the "> " prompt appeared when the system expected
a prompt in the INTERPRET context, whether the previous command aborted
or not.

Also, this allows us to get rid of that ugly FLAGS global variable.

The way is clear for complete stage1 bootstrapping on the RC2014
target!