rework the NIZK Fiat-Shamir Proofs to include extra information in the challenge

[benadida]

This makes Helios provably secure and defends against some particularly advanced attacks.

[adamalexandru4]

@benadida I would like to hear some hints how can I do this. I would like to contribute and the idea to hash the JSON structure you mentioned in the doc looks good from my point of view.

[BenSmyth]

@adamalexandru4 Clarkson, Frink, and I propose a variant of Helios that includes extra information in the challenge (https://eprint.iacr.org/2015/233), we've proven individual- and universal-verifiability, and elsewhere (https://eprint.iacr.org/2015/942) I've proven ballot secrecy, those works are summarised in https://eprint.iacr.org/2018/225.