Review challenge generation

[nikkolasg]

Currently the way the transcript works is done accordingly to the bulletproof usage of Merlin transcript which differs from the regular arkwork. A couple of things to review and adapt:

• it takes 64 bytes of output from the hash function currently, maybe arkwork does it differently
• it tries to see if there is an inverse first, but doesn't return it, leading to two inversion sometimes (because we need the inverse)