Simplifies the retrieval protocol (see `#259 `_ for the discussion). ``PolicyMessageKit`` is renamed to ``MessageKit``. ``Bob.retrieve()`` is renamed to ``retrieve_and_decrypt()``, and its signature is simplified: it only requires the treasure map, Alice's verifying key, and the policy encrypting key. A lower-level ``Bob.retrieve()`` is added that does not decrypt, but only attempts to retrieve the capsule frags.