Reduce boilerplate for implementing alternate versions of Distributions

[SamuelBrand1]

          Nice looks good. As we discussed its a lot of copy and paste so ideally we should have an issue to track changes we can make to reduce this (either here or upstream).

Originally posted by @seabbs in #418 (review)

[SamuelBrand1]

To flesh this out a bit. Implementing SafePoisson and SafeNegativeBinomial was done in a really boilerplate way, involving a lot of copy and paste from Distributions.jl and RandomPoisson.jl.

Possible preferable alternatives:

• A macro based approach which generates the various functions to make a Safe version of a dist e.g.

@make_safe_distr Poisson
# SafePoisson is available

• A traits based approach where we can make a trait-based dispatch without having to redeclare a lot of auxillary functions e.g. something like

@traitdef IsUnsafe{Poisson}
@traitfn rand(d::Poisson) where {Poisson; IsUnsafe{Poisson}} = safe_rand_method(d)