Add some interface functions to support the new Gibbs sampler in Turing #335

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	12m 22s
test (1.6, ubuntu-latest, x64, 1)	13m 16s
test (1.6, macOS-latest, x64, 1)	1s
test (1.6, windows-latest, x64, 1)	12m 21s
test (1, ubuntu-latest, x86, 1)	11m 15s
test (1, ubuntu-latest, x86, 2)	10m 56s
test (1, ubuntu-latest, x64, 1)	11m 54s
test (1, ubuntu-latest, x64, 2)	11m 51s
test (1, macOS-latest, x64, 1)	1s
test (1, macOS-latest, x64, 2)	1s
test (1, windows-latest, x64, 1)	11m 17s
test (1, windows-latest, x64, 2)	10m 49s
test (nightly, ubuntu-latest, x86, 1)	10m 37s
test (nightly, ubuntu-latest, x86, 2)	10m 42s
test (nightly, ubuntu-latest, x64, 1)	10m 33s
test (nightly, ubuntu-latest, x64, 2)	10m 40s
test (nightly, macOS-latest, x64, 1)	1s
test (nightly, macOS-latest, x64, 2)	1s
test (nightly, windows-latest, x64, 1)	10m 3s
test (nightly, windows-latest, x64, 2)	10m 2s
	2h 48m 43s

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