This builds on heaps of work by @chm-diederichs and is basically just
the cherry on top that exposes all of the underlying work that they've
done. The changes from this commit:
- Expose crypto_auth() and crypto_auth_verify().
- Fix 'crypto_auth_hmacsha512_BYTES' typo.
- Change output in crypto_auth_hmacsha512256().
- Change crypto_verify_n() to return booleans.