[noise] Extension spec: Signatures

[Trevor Perrin]

I started an extension spec for basic signatures:
 * sig, sigi, and sigr modifiers
 * s1, sig1 tokens (or other numbers, if used in combination with hfs
or something else)

https://github.com/noiseprotocol/noise_sig_spec/
https://github.com/noiseprotocol/noise_sig_spec/blob/master/output/noise_sig.pdf

This follows the hfs spec in using numbered suffixes to refer to
different types of public-key algorithms that are introduced by the
pattern modifiers:

XXhfs+sig_25519+NewHope+Ed25519_AESGCM_SHA256:
 -> e, e1
 <- e, ekem1, s2, sig2
 -> s2, sig2


This spec lists 23 new patterns applying the sig modifiers to the
fundamental+defered patterns.  These signatures modifiers don't
coexist with any ss/noss modifiers, but should combine easily with the
hfs modifier (as above).


Anyways, this gives us 3 extension specs related to patterns:
 * ss (adding and removing ss token)
 * hfs (adding post-quantum KEMs for more forward-secrecy)
 * sig (adding signatures for authentication)

I think that's a good amount of handshake-pattern stuff to discuss at
RWC and try to finalize in the next few months.

(Aside from the pattern work, I'd like to make progress on the SHO
idea, and after that on NoiseSocket/NoiseLingo/NLS).

Trevor