REFERENCE CODE: VRF for secp256k1

[ChristopherA]

In order to be able to implement certain kinds of architectures that leverage commitments (for instance Key Transparency, Kademilia, CONIKS) we need an implementation of Verifiable Random Functions.

There does exist a C fork of bitcoin-core/secp256k1 at https://github.com/aergoio/secp256k1-vrf that implements VRF to the IETF standard, which likely is close to what we need. There is also one in Rust that is more questionable: https://github.com/debasish-raychawdhuri/vrf_on_secp256k1

See also #88