Finally I had to slightly change the lnotab format to have the right
tracing semantics: the change is that line number increments are now
signed bytes rather than unsigned.

Still, there is a small change in tracing behaviour (see test_trace.py):
the for statement is traced twice in the first loop iteration, that's
because the iterator object is retrieved (GET_ITER) at the beginning of
the loop while the next iterator value is fetched (FOR_ITER) at the end
of the loop. I don't believe this is very painful.

All in all, the whole test suite now passes fine. The performance
numbers are the same as before.