[noise] Revision 7: merged "simpler" branch

[Trevor Perrin]

I cleaned up the text in "simpler" branch and merged it.

 To recap, this includes substantive changes:
  * Encrypting/hashing ephemeral public keys so they are treated the
same as static public keys
  * Simplifying MixKey() so it always does an HMAC(), instead of
special-casing the first time as HASH()
  * Hash handshake ciphertext instead of plaintext, so that if h is
used for signing or other purposes, it can't leak plaintext
information
  * Drop pre-shared symmetric keys, the same thing can be done with
pre-message ephemerals

I think I'm happy with this and will do some trial implementation(s)
in next week or so.

Trevor