[noise] New branch: hkdf

[Jason A. Donenfeld]

In the process of implementing this branch now.
(I think I've implemented nearly every revision of noise!)

One optimization you could make is for the first call to HKDF is instead of:

    temp = KDF(handshake_name, dh_result_data)

Just doing instead:

    temp = HASH(dh_result_data)

Since handshake_name isn't secret. OTOH, there's the key reuse
concerns we discussed prior, so maybe it's best how you have it. Just
a thought.