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