Add lemma `pmf_measurable` #539

Job	Run time
coq	1m 0s
mathcomp	1m 2s
mathcomp-analysis-single	17m 17s
mathcomp-classical	3m 0s
mathcomp-reals	4m 22s
mathcomp-reals-stdlib	1m 33s
mathcomp-analysis	15m 15s
mathcomp-experimental-reals	2m 9s
mathcomp-analysis-stdlib	15m 36s
ssprove	21m 52s
	1h 23m 6s

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