This drops in a copy of a subset of golang.org/x/crypto/poly1305 to implement
Poly1305. Hopefully this will keep them from regression as we rework the record
layer.

Change-Id: Ic1e0d941a0a9e5ec260151ced8acdf9215c4b887
Reviewed-on: https://boringssl-review.googlesource.com/4257
Reviewed-by: Adam Langley <agl@google.com>