Span class. (#3548)

* Add basic Span class based on ISO++20.

* Use Span<Entry const> instead of Inst in SparsePage.

* Add DeviceSpan in HostDeviceVector, use it in regression obj.

tests/cpp/c_api/test_c_api.cc

@@ -59,10 +59,10 @@ TEST(c_api, XGDMatrixCreateFromMat_omp) {
       auto batch = iter->Value();
       for (int i = 0; i < batch.Size(); i++) {
         auto inst = batch[i];
-        for (int j = 0; i < inst.length; i++) {
+        for (int j = 0; i < inst.size(); i++) {
           ASSERT_EQ(inst[j].fvalue, 1.5);
         }
       }
     }
   }
-}
+}