Transcript challenge generation optimizations

[lucasxia01]

We can truncate hashes to be 128 bits, so multiplication is easier.

Alternatively, we can generalize the transcript to take in a num_challenge_bits parameter, and then divide up each hash into num_challenge_bits chunks, so we maximize the number of challenges generated from a hash. In practice, since we want 128 bit security, we can create 2 challenges out of each hash.

Generalize the type of hash we use by taking that in as a parameter as well.

Edit: in the new transcript update, we do not truncate hashes. So one optimization would be to truncate the hash.