This change imports the chacha20poly1305 and poly1305 packages from
x/crypto at 5f4e837b98443e9e7a65072235205993af565d85. These packages
will be used to support the ChaCha20-Poly1305 AEAD in crypto/tls.

Change-Id: I1a38d671ef9aeff3bc41e3924655883d465a5617
Reviewed-on: https://go-review.googlesource.com/30956Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>