* Add protocol buffer messages for Proof and Lemma.
* Implement Protobuf serialization for Proof and Lemma.
* Implement Protobuf unserialization for Proof and Lemma.
* Add write_to_bytes and parse_from_bytes methods to Proof.
* Put the Protobuf-related code behind a feature flag.
* Enable more flags. See #10
* Fix clippy warnings. See #10