C++: IR-based range analysis of addition

[MathiasVP]

This PR adds range analysis for binary addition. The library already provided less-than-optimal lower (resp. upper) bounds for addition when a one of the operands was guaranteed to be negative (resp. positive), and this PR replaces that logic by simple just calling bounded recursively on both operands.

This provides us with many more bounds (since we're now analyzing all additions), and also more precise bounds (since we're no longer just giving a bound based on the sign analysis).

As can be seen in the 1a61864 commit, we're losing quite a few bounds on cases like:

if(5 <= b && b <= 23) {
  int r = 11*b;
  total += r;
  range(total);
}

before, we were able to determine that r was always positive, and thus whatever lower bound we had for total before total += r continued to be valid. Now that we're throwing away the sign analysis, we can't deduce a lower bound here anymore since we still don't compute a range for multiplication (I'll do this in a follow-up PR).

[MathiasVP]

Integration tests are failing due to unrelated Ruby changes. Now fixed.