R 3.6.x Update for Sample Extension

[coatless]

Glancing over the RcppArmadillo code, the Alt RNG setup directly uses R's RNG for uniform (e.g. Rf_runif(). However, the extension code in inst/RcppArmadilloExtension/sample.h does not.

So, likely this will need a patch that mirrors what is available in Base R. c.f. Implementation changes.

• Rejection sampling routine
• Implementation of R_unif_rand() in existing R code
• Walker sampling changes

This will likely be needed with Rcpp's local variant as well designed by @nathan-russell.

[coatless]

Per a quick slack discussion, we'll likely want to rewrite portions of RcppArmadilloExtensions/sample.h to reuse parts of the Sugar functions in Rcpp.