Extend marginalization support to Truncated discrete RVs

[ricardoV94]

#91 Implements automatic logp marginalization for bernoulli, categorical and discrete uniform, as those are the only default discrete univariate RVs with finite support. We could extend it easily to right-truncated discrete RVs so that people can use it for Truncated poisson, binomial, mixtures, etc...)

We could also extend it to right-clipped RVs, but I am not sure how useful that is