Add some interface functions to support the new Gibbs sampler in Turing #328

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	3m 7s
test (1.6, ubuntu-latest, x64, 1)	3m 2s
test (1.6, macOS-latest, x64, 1)	3m 14s
test (1.6, windows-latest, x64, 1)	2m 47s
test (1, ubuntu-latest, x86, 1)	3m 3s
test (1, ubuntu-latest, x86, 2)	3m 1s
test (1, ubuntu-latest, x64, 1)	2m 44s
test (1, ubuntu-latest, x64, 2)	2m 42s
test (1, macOS-latest, x64, 1)	3m 9s
test (1, macOS-latest, x64, 2)	3m 9s
test (1, windows-latest, x64, 1)	3m 1s
test (1, windows-latest, x64, 2)	3m 4s
test (nightly, ubuntu-latest, x86, 1)	2m 32s
test (nightly, ubuntu-latest, x86, 2)	2m 35s
test (nightly, ubuntu-latest, x64, 1)	2m 23s
test (nightly, ubuntu-latest, x64, 2)	2m 9s
test (nightly, macOS-latest, x64, 1)	3m 12s
test (nightly, macOS-latest, x64, 2)	3m 6s
test (nightly, windows-latest, x64, 1)	3m 11s
test (nightly, windows-latest, x64, 2)	3m 11s
	58m 22s

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