Extracted StringReader and StringWriter from JsonParser

Split `Internals/` folder into `Data/`, `Deserialization/`, `Serialization/`

include/ArduinoJson/Data/JsonVariantDefault.hpp

@@ -0,0 +1,26 @@
+// Copyright Benoit Blanchon 2014-2016
+// MIT License
+//
+// Arduino JSON library
+// https://github.com/bblanchon/ArduinoJson
+// If you like this project, please add a star!
+
+#pragma once
+
+namespace ArduinoJson {
+namespace Internals {
+
+template <typename T>
+struct JsonVariantDefault {
+  static T get() {
+    return T();
+  }
+};
+
+template <typename T>
+struct JsonVariantDefault<const T> : JsonVariantDefault<T> {};
+
+template <typename T>
+struct JsonVariantDefault<T&> : JsonVariantDefault<T> {};
+}
+}