[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/xgboost.R

@@ -105,5 +105,5 @@ NULL
 #' @importFrom graphics title
 #' 
 #' @import methods
-#' @useDynLib xgboost
+#' @useDynLib xgboost, .registration = TRUE
 NULL