Kolmogorov distribution and KS tests

Adds the Kolmogorov distribution (with mean, var, cdf and ccdf). Used in the large-sample Kolmogorov-Smirnov test (for small samples, the exact distribution can be computed, but this is a bit trickier).

I've also added KS tests to the `test/univariate_stats.jl` to check the rng vs cdf of the distributions. This adds a little bit of time, but I think it's worth it, particularly as we're looking to rely less on rmath.

p.s. I like the new constants by the way...