Uses the pairing library written by ZCash project:
https://electriccoin.co/blog/pairing-cryptography-in-rust/
Originally pairing library, now renamed to bls12_381
TODO:
* Cleanup code
* Organize tuples into data structures and type aliases
* Abstract common operations (witness - challenge * response),
computing challenges, ...
* Finish sum trait for G1/G2... create new trait for shared stuff make pullreq
* Organize into new API with structs
* Add public attributes
* Properly find terms for each term
* Move shared state and config into a coconut struct
* Document API and make unit tests
* Create ProofBuilder class which is extendable with custom phi value from paper
* Support for other curves
* Optimizations