Update NegativBinomial parametrizations with more stable Distributions.jl backend

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JuliaStats/Distributions.jl#1536 is proposing different parametrizations of the NegativeBinomial likelihood, particularly the scale-shape parametrization that corresponds to the NBParamMean types introduced in #80. Once JuliaStats/Distributions.jl#1536 is released as part of Distributions.jl, we should update our code accordingly (specifically, we should be able to remove the _nb_mean_excessvar_to_r_p code).