[noise] Revision 30, take 2

[Trevor Perrin]

I scaled back ambitions for revision 30:

 * Removed sections on semi-static patterns and variant patterns for
pipes.  In light of discussions like [1] and [2] I think there's
enough complexity that we should defer these to a future revision,
where we have a dedicated section on Pipes and can analyze this in
more detail.

 * Added a note to reserve the maximum cipher nonce 2^64-1 for future
use, since I'm still thinking of something like [3].


https://github.com/noiseprotocol/noise_spec/compare/master...rev30
https://github.com/noiseprotocol/noise_spec/blob/rev30/output/noise.pdf


I'd like to publish revision 30 Friday, so please review and send
feedback.  The main additions are:

 * "dependent patterns" to cover PSKs in combination with fallback patterns.

 * "transformations" like "noidh" and "fallback", since this seems
like a good way of handling new patterns.

 * "e, s" instead of "s, e" for pre-messages, to make patterns more
consistent and fallback transformations easier to describe

 * n = 2^64-1 reserved for future use

 * better acknowledgements, rationale, etc


[1] https://moderncrypto.org/mail-archive/noise/2016/000686.html
[2] https://moderncrypto.org/mail-archive/noise/2016/000689.html
[3] https://moderncrypto.org/mail-archive/noise/2016/000659.html