audio/lpcnet: New port: Neural Net speech coding

Needed for freedv update see:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=264769

Experimental version of LPCNet that has been used to develop FreeDV 2020

• a HF radio Digial Voice mode for over the air experimentation with

Neural Net speech coding. Possibly the first use of Neural Net speech
coding in real world operation.