[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.R

@@ -37,6 +37,6 @@ xgb.save <- function(model, fname) {
          if (inherits(model, "xgb.DMatrix")) " Use xgb.DMatrix.save to save an xgb.DMatrix object." else "")
   }
   model <- xgb.Booster.complete(model, saveraw = FALSE)
-  .Call("XGBoosterSaveModel_R", model$handle, fname[1], PACKAGE = "xgboost")
+  .Call(XGBoosterSaveModel_R, model$handle, fname[1])
   return(TRUE)
 }