Add some interface functions to support the new Gibbs sampler in Turing #296

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	6m 54s
test (1.6, ubuntu-latest, x64, 1)	5m 2s
test (1.6, macOS-latest, x64, 1)	6m 42s
test (1.6, windows-latest, x64, 1)	13m 45s
test (1, ubuntu-latest, x86, 1)	31m 30s
test (1, ubuntu-latest, x86, 2)	31m 11s
test (1, ubuntu-latest, x64, 1)	5m 16s
test (1, ubuntu-latest, x64, 2)	4m 53s
test (1, macOS-latest, x64, 1)	5m 59s
test (1, macOS-latest, x64, 2)	6m 15s
test (1, windows-latest, x64, 1)	6m 6s
test (1, windows-latest, x64, 2)	5m 39s
test (nightly, ubuntu-latest, x86, 1)	8m 10s
test (nightly, ubuntu-latest, x86, 2)	7m 10s
test (nightly, ubuntu-latest, x64, 1)	5m 51s
test (nightly, ubuntu-latest, x64, 2)	5m 30s
test (nightly, macOS-latest, x64, 1)	7m 17s
test (nightly, macOS-latest, x64, 2)	6m 23s
test (nightly, windows-latest, x64, 1)	6m 16s
test (nightly, windows-latest, x64, 2)	6m 4s
	3h 1m 53s

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