Better encapsulation and safety guarantee for Entries

[YaxinCheng]

• Functions in Entry is safe now because only the latest Entry will be accessible.
  • In the non-lending-iter feature scenario, every Entry is passed into a callback closure as mutable references. This prevents the Entrys from being moved out of the closure.
  • In the lending-tier feature scenario, the lending iterator guarantees the references cannot be moved.
• Reading an entry file content more than once will silently return. This matches with the behaviour when reading a file from EOF.