speeding up IntInf?

[Matthew Fluet]

> Henry, it occurs to me that at the time you wrote the implementation
> of IntInf, we didn't implement integer arithmetic raising Overflow.  I
> was wondering if it would make sense (for you) to go back through the
> implementation and look at places where you could use Overflow to
> speed stuff up.

We might also think about removing the dependency on the flattening-pass
for the correctness of the IntInf implementation.  With all of the
explicit array and frontier management, this probably isn't that hard.