new opmsg trickery

Theres two news to the opmsg:

First: I added brainkey personas, a similar thing to
BTC brainwallets. If you use these, you do not need
to send your pubkeys around anymore. It's an experimental
feature. The key is derived via PBKDF2 function with HMAC
with a lot of iterations and salt, so to prevent large
scale (or huge-scale) dictionary attacks. True, I could have used
memory-hard functions for that but a) Argon2 or friends
are not yet part of OpenSSL and b) if I understand correctly,
you can still use it on massive paralleled ASCICs if you
accept additional computation cost for re-creating matrices
on the fly. So a+b let me chose PBKDF2 over Argon2.
Brainkey personas are also deniable personas, which underlines
the concept of linked/deniable personas.

Second: you can im/export your persona keys as QR codes
now, to make it more convenient to exchange pubkeys
due to ubiquitous (what a word!) dumb phones that can send around screenshots.
Its inside its own git repo that you can find here.
Most of the effort was finding and integrating small
footprint QR encode/decoding libraries.