Increase efficiency of EnumeratedDistribution#probability

There are lots of other low hanging fruit in the distribution package but unfortunately this is the only one I got to that day.

In the EnumeratedDistribution case, the probability() method is currently O(N) where N denotes number of random variables, even though the random variables are fixed / known. This change makes probability() O(1). The unlikely worst case scenario now consumes slightly more than 2x the memory, but I do not think this would be an issue in the vast majority of cases.

Original PR (incorrectly against master) here.