Add some interface functions to support the new Gibbs sampler in Turing #316

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	3m 1s
test (1.6, ubuntu-latest, x64, 1)	2m 49s
test (1.6, macOS-latest, x64, 1)	3m 3s
test (1.6, windows-latest, x64, 1)	3m 2s
test (1, ubuntu-latest, x86, 1)	2m 57s
test (1, ubuntu-latest, x86, 2)	2m 57s
test (1, ubuntu-latest, x64, 1)	2m 39s
test (1, ubuntu-latest, x64, 2)	2m 44s
test (1, macOS-latest, x64, 1)	3m 0s
test (1, macOS-latest, x64, 2)	3m 0s
test (1, windows-latest, x64, 1)	2m 55s
test (1, windows-latest, x64, 2)	2m 52s
test (nightly, ubuntu-latest, x86, 1)	3m 0s
test (nightly, ubuntu-latest, x86, 2)	3m 3s
test (nightly, ubuntu-latest, x64, 1)	2m 57s
test (nightly, ubuntu-latest, x64, 2)	3m 0s
test (nightly, macOS-latest, x64, 1)	3m 1s
test (nightly, macOS-latest, x64, 2)	3m 2s
test (nightly, windows-latest, x64, 1)	2m 59s
test (nightly, windows-latest, x64, 2)	2m 59s
	59m 0s

Provide feedback