R interface

[WardBrian]

Mirroring #1

A basic version that just allows Stan calls is trivial based on the code in #78.

Allowing log densities specified in R would require a separate shim library, likely based on Rcpp. Existing examples of this kind of thing can be found:

• https://github.com/eddelbuettel/rcppde/blob/master/src/evaluate.h
• https://github.com/andrjohns/StanEstimators/blob/main/src/include/estimator/estimator_ext_header.hpp
• https://gallery.rcpp.org/tags/function/

Note that this would require linking with R's code and that resulting code would become GPL licensed.

We may also be able to get some help from @andrjohns or @jgabry