Using cmdstanr and cmdstanpy instead

[JaeyeongYang]

Recently, Stan developers released new packages, cmdstanr and cmdstanpy, which provide direct interfaces to cmdstan. By using so, not only can we use the same interface for stan in both R and Python (so as to make development easier), but we can also utilize the latest stan with new good features.

It might conflict with issue #127, so it'd be better to decide whether to keep the current dependencies on rstan and pystan or to move on to cmdstanr and cmdstanpy.

[JaeyeongYang]

It might be great to consider those as next backends, like other packages with Stan backends (e.g., brms, prophet) does, but we cannot migrate into them yet due to the fact that cmdstanr is still in beta and not released in cran.