This mostly adds the infrastructure for writing various forms of
packets as well as reading them. Adding symmetric encryption support
was simply an easy motivation.

There's also one brown-paper-bag fix in here. Previously I had the
conditional for the MDC hash check backwards: the code was checking
that the hash was *incorrect*. This was neatly counteracted by another
bug: it was hashing the ciphertext of the OCFB prefix, not the
plaintext.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4564046