[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.DMatrix.save.R

@@ -18,6 +18,6 @@ xgb.DMatrix.save <- function(dmatrix, fname) {
   if (!inherits(dmatrix, "xgb.DMatrix"))
     stop("dmatrix must be xgb.DMatrix")
   
-  .Call("XGDMatrixSaveBinary_R", dmatrix, fname[1], 0L, PACKAGE = "xgboost")
+  .Call(XGDMatrixSaveBinary_R, dmatrix, fname[1], 0L)
   return(TRUE)
 }