Address review concerns from #164.

Address review concerns from #164.

1. Remove recommendation for 512-bit BLAKE2b.
   32 bytes is enough, and it's not like we offer EC functions of a
   higher security level either.
   The text added in 628f027 already does enough to recommend proper
   hash output lengths.
2. Bump .Dd date in crypto_poly1305.3monocypher.
3. crypto_verify16 add "byte by byte" for both accuracy of how a MAC
   with a variable-time comparison function will be found and
   for dramatic reasons because it sounds like doom is slowly
   approaching, byte by byte.