Implement zksnark prover in javascript

[barryWhiteHat]

Right now we have to call the cpp code to make a zksnark proof before withdrawing. It would be better if this was done in javascript so that cpp only needs to be called for parameter generation.

Note this will involve loading the verification key which is ~800MB

[barryWhiteHat]

Or call the cpp prove function from javascript

[barryWhiteHat]

Right now i call the cpp funciton from python. But I would be nice to have a pure python implementation. But that is unlikely. ZoKrates are doing some work with compileing witness generation to WAS which could work for us here.