Rename axolotlpp as olm to avoid confusion with Axolotl-the-spec and Axolotl-the-OWS-libraries at moxie's request

Pass the message body to decrypt_max_plaintext_length so we can get a more accurate estimate, rename encrypt_max_output_length to encrypt_output_length and change the api to return the exact number of bytes needed to hold the message

git-subtree-dir: lib/ed25519
git-subtree-split: 1fc4a2ff69fe111875bd5efcc8523e40b18cf673