Implement zero-knowledge proofs for public values

[olabini]

The OTRv4 Prekey Server specification has been updated with support for verifying public values. We should implement this.

• [X] implement proofs
• [X] add serialization of proofs
• [x] add proofs to publication message ser and deser
• [x] add proofs to MAC
• [x] add verification of proofs to validation of publication message

[olabini]

Fixed.