Add some interface functions to support the new Gibbs sampler in Turing #298

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	6m 47s
test (1.6, ubuntu-latest, x64, 1)	4m 59s
test (1.6, macOS-latest, x64, 1)	5m 46s
test (1.6, windows-latest, x64, 1)	15m 27s
test (1, ubuntu-latest, x86, 1)	32m 36s
test (1, ubuntu-latest, x86, 2)	30m 13s
test (1, ubuntu-latest, x64, 1)	5m 20s
test (1, ubuntu-latest, x64, 2)	4m 41s
test (1, macOS-latest, x64, 1)	5m 48s
test (1, macOS-latest, x64, 2)	6m 13s
test (1, windows-latest, x64, 1)	5m 26s
test (1, windows-latest, x64, 2)	5m 40s
test (nightly, ubuntu-latest, x86, 1)	6m 57s
test (nightly, ubuntu-latest, x86, 2)	5m 49s
test (nightly, ubuntu-latest, x64, 1)	4m 36s
test (nightly, ubuntu-latest, x64, 2)	4m 33s
test (nightly, macOS-latest, x64, 1)	5m 39s
test (nightly, macOS-latest, x64, 2)	4m 54s
test (nightly, windows-latest, x64, 1)	5m 9s
test (nightly, windows-latest, x64, 2)	5m 11s
	2h 51m 44s

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