Uniform distribution over the standard simplex

I prepared `Data.Random.Distribution.Simplex`, which provides a class of uniform distribution over the standard simplex. The standard simplex of dimension _k_ is defined by { (_x_0_, _x_1_, ..., _x_k_) | _x_i_ >= 0 for all _i_, sum _x_i_ = 1 }. Using this module, one can get a random sample of positive real values where its sum is 1. This is useful when, for example in a context of Bayesian statistics, one wants to initialize a prior (discrete) distribution uniformly at random.