Add some interface functions to support the new Gibbs sampler in Turing #332

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	32s
test (1.6, ubuntu-latest, x64, 1)	34s
test (1.6, macOS-latest, x64, 1)	30s
test (1.6, windows-latest, x64, 1)	26s
test (1, ubuntu-latest, x86, 1)	25s
test (1, ubuntu-latest, x86, 2)	24s
test (1, ubuntu-latest, x64, 1)	22s
test (1, ubuntu-latest, x64, 2)	22s
test (1, macOS-latest, x64, 1)	28s
test (1, macOS-latest, x64, 2)	38s
test (1, windows-latest, x64, 1)	28s
test (1, windows-latest, x64, 2)	28s
test (nightly, ubuntu-latest, x86, 1)	29s
test (nightly, ubuntu-latest, x86, 2)	30s
test (nightly, ubuntu-latest, x64, 1)	38s
test (nightly, ubuntu-latest, x64, 2)	36s
test (nightly, macOS-latest, x64, 1)	28s
test (nightly, macOS-latest, x64, 2)	28s
test (nightly, windows-latest, x64, 1)	28s
test (nightly, windows-latest, x64, 2)	25s
	9m 39s

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