Feature/issue 3121 beta neg binomial rng #4457

Job	Run time
fwd tests and non-fun mix tests	1h 1m 14s
mix/fun tests 2	1h 31m 24s
mix/fun tests 1	2h 9m 40s
prim and rev tests	2h 47m 1s
	7h 29m 19s

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