Encrypted Messages + Nonces

Currently all of the ``encrypt()`` functions accept a nonce and return ciphertext and conversely all of the ``decrypt()`` functions accept a nonce and return a plaintext.

Should the ciphertext instead include the nonce to make things simpler for end users? If so should we do it similarly to ``SignedMessage`` where we return a bytes subclass with the nonce and ciphertext together with attributes to access them separately?