Add IO utilities. (#5091)

* Add fixed size stream for reading model stream.
* Add file extension.

tests/cpp/common/test_io.cc

@@ -0,0 +1,43 @@
+/*!
+ * Copyright (c) by XGBoost Contributors 2019
+ */
+#include <gtest/gtest.h>
+#include "../../../src/common/io.h"
+
+namespace xgboost {
+namespace common {
+TEST(IO, FileExtension) {
+  std::string filename {u8"model.json"};
+  auto ext = FileExtension(filename);
+  ASSERT_EQ(ext, u8"json");
+}
+
+TEST(IO, FixedSizeStream) {
+  std::string buffer {"This is the content of stream"};
+  {
+    MemoryFixSizeBuffer stream(static_cast<void *>(&buffer[0]), buffer.size());
+    PeekableInStream peekable(&stream);
+    FixedSizeStream fixed(&peekable);
+
+    std::string out_buffer;
+    fixed.Take(&out_buffer);
+    ASSERT_EQ(buffer, out_buffer);
+  }
+
+  {
+    std::string huge_buffer;
+    for (size_t i = 0; i < 512; i++) {
+      huge_buffer += buffer;
+    }
+
+    MemoryFixSizeBuffer stream(static_cast<void*>(&huge_buffer[0]), huge_buffer.size());
+    PeekableInStream peekable(&stream);
+    FixedSizeStream fixed(&peekable);
+
+    std::string out_buffer;
+    fixed.Take(&out_buffer);
+    ASSERT_EQ(huge_buffer, out_buffer);
+  }
+}
+}  // namespace common
+}  // namespace xgboost