Add Cholesky optimization to LKJ priors when available.

[itsdfish]

Hi Rob,

It turns out that there is an ongoing effort to add Cholesky factorization to the LKJ prior distribution. This will improve the efficiency and numerical stability of the multilevel models. I am willing to add those improvements when they become available.

[goedman]

Awsome Chris, thanks a lot. Just wished I had more time to spend on the Turing side of things!