[noise] Symmetric-crypto overhaul and stateful hashing

[Trevor Perrin]

Another bug is that we'd need to encode the Noise protocol name the
same way as before (padded to HASHLEN or hashed to HASHLEN), so new
and old names can't be confused with each other.  So this:

transcript = encode(name) || len(prologue) || prologue || e || es

Instead of this:

transcript = len(name) || name || len(prologue) || prologue || e || es

Trevor