Verifiable delay function

[xhliu]

References

https://github.com/kilic/evmvdf#references

Reference impl

https://github.com/witnet/vrf-solidity
https://www.justinsilver.com/technology/cryptocurrency/provable-randomness-with-vdf/
https://github.com/kilic/evmvdf
https://github.com/0xProject/VDF