Add some interface functions to support the new Gibbs sampler in Turing #292

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	6m 44s
test (1.6, ubuntu-latest, x64, 1)	5m 2s
test (1.6, macOS-latest, x64, 1)	5m 59s
test (1.6, windows-latest, x64, 1)	12m 39s
test (1, ubuntu-latest, x86, 1)	30m 42s
test (1, ubuntu-latest, x86, 2)	31m 7s
test (1, ubuntu-latest, x64, 1)	5m 21s
test (1, ubuntu-latest, x64, 2)	4m 46s
test (1, macOS-latest, x64, 1)	5m 48s
test (1, macOS-latest, x64, 2)	5m 27s
test (1, windows-latest, x64, 1)	6m 2s
test (1, windows-latest, x64, 2)	5m 39s
test (nightly, ubuntu-latest, x86, 1)	8m 11s
test (nightly, ubuntu-latest, x86, 2)	7m 28s
test (nightly, ubuntu-latest, x64, 1)	5m 37s
test (nightly, ubuntu-latest, x64, 2)	5m 23s
test (nightly, macOS-latest, x64, 1)	6m 33s
test (nightly, macOS-latest, x64, 2)	6m 20s
test (nightly, windows-latest, x64, 1)	6m 33s
test (nightly, windows-latest, x64, 2)	6m 33s
	2h 57m 54s

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