[R] native routines registration (#2290)

* [R] add native routines registration

* c_api.h needs to include <cstdint> since it uses fixed width integer types

* [R] use registered native routines from R code

* [R] bump version; add info on native routine registration to the contributors guide

* make lint happy

R-package/R/xgb.save.raw.R

@@ -19,5 +19,5 @@
 #' @export
 xgb.save.raw <- function(model) {
   model <- xgb.get.handle(model)
-  .Call("XGBoosterModelToRaw_R", model, PACKAGE = "xgboost")
+  .Call(XGBoosterModelToRaw_R, model)
 }