Function encrypt_private_message 

pub fn encrypt_private_message<M>(
    additionally_authenticated_data: &[u8],
    authentication_policy: AuthenticationPolicy,
    chacha_rounds: ChaChaRounds,
    message: &mut M,
    receiver_public_key: &PublicKey,
    sender_keypair: &Ed25519KeyPair,
) -> Result<(), PrivateMessageError>
where
    M: AsRef<[u8]> + AsMut<[u8]> + Extend<u8>,

Encrypt private message