There is now no more actual code in stable ABI, only references.
This makes refactoring of this code much easier. For example,
changing IY to BC as the IP register.
Previously, it was impossible to cross-compile Collapse OS from a
binary-offsetted Collapse OS because stable ABI wordrefs would have
a wrongly offsetted address.
This solves the problem by replacing those wordrefs by direct,
hardcoded stable ABI offset references.
It was useful when we still had the relinker, but now it has no use.
I was waiting a bit to see if the distinction would be useful again,
but it seems like it won't.
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.
I'm planning on going back to 8-bit branching. 16-bit br cells incur
a non-negligible penalty and, while at first 64 words (128 bytes
forward or backward) seemed a bit limiting, I now don't see why one
would ever construct such a big branch. It would be un-forthy.
Also, I looked at using BC instead of IY to hold IP and the transition
would be a lot easier with 8-bit branching.
In this commit, all I do is add overflow checks in IF. The mechanic
below doesn't change. I'll give myself some time to think it over so
that I avoid yet another back and forth.
