Use cmdstan instead of stan to reduce compile time

[hongyuanjia]

cmdstanr provides a clean interface to Stan services with minimal dependencies.

There is user observing significant time saving:

Simple linear regression with 5 observations:

system.time for rstan was (85.876   5.269  47.514).
Same model in CmdStanR (compilation + sampling): 
system.time ( 1.522   0.321   2.435 )