* Added is_fast property on BatchEncoding to indicate if the object comes from a Fast Tokenizer.
* Added __get_state__() & __set_state__() to be pickable.
* Correct tokens() return type from List[int] to List[str]
* Added unittest for BatchEncoding pickle/unpickle
* Added unittest for BatchEncoding is_fast
* More careful checking on BatchEncoding unpickle tests.
* Formatting.
* is_fast should assertTrue on Rust tokenizers.
* Ensure tensorflow has correct way of checking array_equal
* More formatting.