[noise] Extension spec: Static-Static Pattern Modifiers

[Trevor Perrin]

I wrote up the "ss" and "noss" modifiers Justin and I worked out a few
months ago:

https://github.com/noiseprotocol/noise_ss_spec/blob/master/output/noise_ss.pdf
https://github.com/noiseprotocol/noise_ss_spec

These give us more control over adding/removing static-static DH:  It
can be added for more resilience against ephemeral-key compromise; or
can be removed from patterns like IK if you don't want to spend a DH
for these benefits of 0-RTT authentication and
ephemeral-key-compromise resilience.

Justin did most of the heavy lifting in figuring these out.  This
follows his earlier spec, except:
 * Omits one-way patterns with "noss", since X and K with noss
basically degrade to N.
 * Omits the description of how noss/ss interact with the spec's
pattern-generation rules, since I think these are easy enough to
understand as pattern transformations.

Trevor