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/common/test_host_device_vector.cu

@@ -0,0 +1,22 @@
+/*!
+ * Copyright 2018 XGBoost contributors
+ */
+
+#include <gtest/gtest.h>
+#include "../../../src/common/host_device_vector.h"
+#include "../../../src/common/device_helpers.cuh"
+
+namespace xgboost {
+namespace common {
+
+TEST(HostDeviceVector, Span) {
+  HostDeviceVector<float> vec {1.0f, 2.0f, 3.0f, 4.0f};
+  vec.Reshard(GPUSet{0, 1});
+  auto span = vec.DeviceSpan(0);
+  ASSERT_EQ(vec.Size(), span.size());
+  ASSERT_EQ(vec.DevicePointer(0), span.data());
+}
+
+}  // namespace common
+}  // namespace xgboost
+