Added GetBytes to IStorageType and implemented it in Relation

* Added GetBytes() method to IStorageType since every storage type
  needs to provide its bytes to be written
* Implemented GetBytes() method in Relation since it implements IStorageType
* Earlier Page implemented Iterator<T> as Iterator<Tuple>, changed it to
  Iterator<IStorageType> so that it can save even types of Relation and others
* Page now copies the StorageTypes value to page before saving the size and the bytes