Extract string view. (#7416)

* Add equality operators.
* Return a view in substr.
* Add proper iterator types.

tests/cpp/common/test_string_view.cc

@@ -0,0 +1,28 @@
+/*!
+ * Copyright (c) by XGBoost Contributors 2021
+ */
+#include <gtest/gtest.h>
+#include <xgboost/string_view.h>
+#include <string_view>
+namespace xgboost {
+TEST(StringView, Basic) {
+  StringView str{"This is a string."};
+  std::stringstream ss;
+  ss << str;
+
+  std::string res = ss.str();
+  ASSERT_EQ(str.size(), res.size());
+  ASSERT_TRUE(std::equal(res.cbegin(), res.cend(), str.cbegin()));
+
+  auto substr = str.substr(5, 2);
+  ASSERT_EQ(substr.size(), 2);
+
+  ASSERT_EQ(StringView{"is"}.size(), 2);
+  ASSERT_TRUE(substr == "is");
+  ASSERT_FALSE(substr != "is");
+  ASSERT_FALSE(substr == "foobar");
+  ASSERT_FALSE(substr == "i");
+
+  ASSERT_TRUE(std::equal(substr.crbegin(), substr.crend(), StringView{"si"}.cbegin()));
+}
+}  // namespace xgboost