Poly1305 message authentication code

Produce a Poly1305 MAC for the provided message, given the provided key.

Per RFC8439: the key, which is essentially a one-time key, should be unique, and MUST be unpredictable for each invocation.

The key must be exactly 256 bits in length. Providing an invalid key will cause the function to throw an ErrorCall exception.

>>> mac "i'll never use this key again!!!" "a message needing authentication"
"O'\231Z\224\149\148\246\203[}\210\203\b\200\207"