Exact Math: Exact computation in Swift

Currently, the package only has a single module, RationalModule. It provides Rational<IntegerType> for performing rational arithmetic with as much care as possible to avoid overflow. It is generic over any type conforming to SignedInteger & FixedWidthInteger.

The package is still not production ready. I haven’t tested the performance as much as I should, and the API may slightly change, but it shouldn’t be anything drastic.