Allow BatchEncoding to be pickled
Overrides the methods ___get_state()___ & ___set_state()___ to (respectively) export the content of the underlying `data` dictionnary and - if defined - the content of `encodings`.

Unittests added to covert the serialization & deserialization of all the exported properties.
 
Block until new release of **tokenizers**
Issues seems related to pretokenized input. Checking with @n1t0 what changed in-between 👍 <|||||>Indeed, I think we'll have to merge https://github.com/huggingface/transformers/pull/4510 before this.<|||||>Closing in favor of #5039