[noise] Extension Spec: SHO (Stateful Hash Objects)

[Trevor Perrin]

I wrote up the SHO idea for stateful hashing, along with a "SHOE"
extension (SHO plus encryption), for supporting things like STROBE.

https://github.com/noiseprotocol/sho_spec/blob/master/output/sho.pdf


This spec isn't Noise-specific but just trying to get the API down,
and proposed constructions for SHO/SHA256, SHO/SHAKE128, SHO/BLAKE2s,
etc.

An additional spec will be needed to integrate this into Noise, but I
think this idea is generally useful and needs analysis on its own, so
should have a document of its own.

Feedback welcome!


Trevor