Decide how to handle cdf for categorial RVs

One option is to force all categories to be comparable, such as with using pairs `(order, value)`. Another possible solution is not to implement `cdf` for general RVs, and then there's a possibility to implement a mid-class that is "not-so-general" whose values have to be comparable, and in fact, have to be linearly-ordered.

I believe that I like the most the last idea.
