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.

src/common/host_device_vector.cc

@@ -53,6 +53,11 @@ GPUSet HostDeviceVector<T>::Devices() const { return GPUSet::Empty(); }
 template <typename T>
 T* HostDeviceVector<T>::DevicePointer(int device) { return nullptr; }
 
+template <typename T>
+common::Span<T> HostDeviceVector<T>::DeviceSpan(int device) {
+  return common::Span<T>();
+}
+
 template <typename T>
 std::vector<T>& HostDeviceVector<T>::HostVector() { return impl_->data_h_; }