add StringifyHelper.cs

* IStringify<T>, StringifyConverter<T>
StringifyConverter: replace default(T) to new T()

* StringifyConverter
* replace default(T) to new T(). (when T is reference type, default(T)
is null)
* MyData2
* replace MyData2Converter to StringifyConverter<MyData2>, and delete
MyData2Converter.cs.