(#15) Reverts several commits, doing away with the ExpressionModule and the Composed class.
It just wasn't a practical solution to the decimal precision problem. It would be better to
just use Scala's BigDecimal. However, we can still use Fraction to represent Slope. So this
revert puts us back when the FractionModule was nice, elegant, and simple.

This reverts commit 874a8629842199d73a7ab856532759ab13372c8e.
Moved FractionModule and LineModule into game.util.math and added comparison operators to Fraction