Bijector for product distribution #548

Job	Run time
test (1.6, ubuntu-latest, x64, ForwardDiff)	3m 41s
test (1.6, ubuntu-latest, x64, Tracker)	3m 24s
test (1.6, ubuntu-latest, x64, ReverseDiff)	3m 38s
test (1.6, ubuntu-latest, x64, Zygote)	4m 30s
test (1.6, macOS-latest, x64, ForwardDiff)	6m 32s
test (1.6, macOS-latest, x64, Tracker)	4m 8s
test (1.6, macOS-latest, x64, ReverseDiff)	6m 50s
test (1.6, macOS-latest, x64, Zygote)	5m 26s
test (1, ubuntu-latest, x64, ForwardDiff)	2m 22s
test (1, ubuntu-latest, x64, Tracker)	2m 17s
test (1, ubuntu-latest, x64, ReverseDiff)	2m 24s
test (1, ubuntu-latest, x64, Zygote)	2m 46s
test (1, macOS-latest, x64, ForwardDiff)	4m 50s
test (1, macOS-latest, x64, Tracker)	3m 12s
test (1, macOS-latest, x64, ReverseDiff)	2m 54s
test (1, macOS-latest, x64, Zygote)	3m 32s
	1h 2m 26s

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