Add some interface functions to support the new Gibbs sampler in Turing #301

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	6m 39s
test (1.6, ubuntu-latest, x64, 1)	4m 47s
test (1.6, macOS-latest, x64, 1)	6m 5s
test (1.6, windows-latest, x64, 1)	12m 53s
test (1, ubuntu-latest, x86, 1)	31m 22s
test (1, ubuntu-latest, x86, 2)	29m 49s
test (1, ubuntu-latest, x64, 1)	5m 14s
test (1, ubuntu-latest, x64, 2)	4m 59s
test (1, macOS-latest, x64, 1)	5m 53s
test (1, macOS-latest, x64, 2)	5m 53s
test (1, windows-latest, x64, 1)	5m 30s
test (1, windows-latest, x64, 2)	5m 29s
test (nightly, ubuntu-latest, x86, 1)	8m 54s
test (nightly, ubuntu-latest, x86, 2)	7m 31s
test (nightly, ubuntu-latest, x64, 1)	5m 55s
test (nightly, ubuntu-latest, x64, 2)	5m 50s
test (nightly, macOS-latest, x64, 1)	1s
test (nightly, macOS-latest, x64, 2)	7m 13s
test (nightly, windows-latest, x64, 1)	11m 33s
test (nightly, windows-latest, x64, 2)	11m 31s
	3h 3m 1s

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