Explore - support more fields

[WizardOfMenlo]

Currently, the implementation uses a 256bit field. Changing to a 128bit field should be an easy performance win. Using a 32/64bit field and a 128 bit extension is very important for the prover, but on the verifier the added complexity of the extension might offset some of the benefits and generally requires more thought.

Ideally, the implementation should be generic across choice of field but I am not sure how hard this is in Solidity.