Arbint now does sign in division correctly

The tester still sometimes picks up a fith Rational<Arbint> /= operation though.
Rational<Arbint> now passes realops.test
digit_t is now unsigned, asm division for 1 digit

We now do the "two's complement" in SubBasic by hand. It is maybe even
correct.
Maybe this is more correct. realops likes it.
More debugging, harder realops test
Arbint subtraction should work* now

Yes I was being clever and resizing if sub.m_digits.size() > this->m_digits.size()
But I forgot about when this->m_digits.size() > sub.m_digits.size()

Passes tests/realops.test now!

On the other hand, CPU rendering of Beziers doesn't work anymore.
I think it's just drawing two lines instead of lots of them.