Improve Base.show for hash functions

[kernelmethod]

The output of Base.show is pretty ugly right now for hash functions, e.g. show(hashfn::MonteCarloHash) will display all of the coefficients for the hash function. Base.show should be implemented for all hash functions to give a concise summary of the hash function (e.g. the similarity metric, the number of hash functions bundled together, and so on).