Add JubJub curve to stdlib to support signature verification on curve bl12_381

[alv-around]

Abstract

Zokrates supports proof generation based on the bl12_381 curve. However, in order verify a signature on the bl12_381 a user have to implement the jubjub embedded curve from scratch. There is also

Motivation

Similarly to the baby jujub, the adding an instance of the jubjub curve in the stdlib would simplify the signature verification for the curve bl12_381.

Specification

Add jubjubParams.zok in the ecc package of zokrates_stdlib

Backwards Compatibility

This feature should not introduce any backward compatibility. Users who would like to use this feature would have to update Zokrates to a newer version, or copy and edit the source code according to their local version.

[alv-around]

@dark64 I was going to work on this feature, but I see you have come far already. I can overtake if you are currently working on other things, if not let me know how I can support.