Improve numerical stability of binomial_logit_lpmf #3573

Job	Run time
fwd tests and non-fun mix tests	53m 50s
fwd tests and non-fun mix tests	53m 50s
prim and rev tests	3h 27m 58s
prim and rev tests	3h 27m 58s
mix/fun tests 1	1h 23m 45s
mix/fun tests 1	1h 33m 30s
mix/fun tests 2	2h 35m 18s
mix/fun tests 2	2h 35m 18s
	16h 51m 27s

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