Added secure channel protocols (experimental)

Added secure channel protocols (experimental)

At long last, the NaCl family of crypto libraries is gaining direct
support for secure channels.

Up until now, the choices were basically invent our own protocol, or
give up and use a TLS library, thus voiding the usability improvements
of NaCl libraries.

Now we have a solution.  It's still a bit experimental, it's not yet
documented, but it's there.  And soon, we will finally be able to shift
the cryptographic right answer for secure channels away from TLS, and
towards the NaCl family.  Or perhaps just Monocypher, if for some reason
Libsodium doesn't follow suit. :-)