Generalize fields and curves

Currently, there are separate folders and files for bn128 and bls-12-381, with ~90% duplicated code.

It should be possible to generalize this more, creating a generic prime field class much like there is a semi-generic FQP class, as well as generic elliptic curves, and then only have separate files for the two existing curve types to define curve parameters and twist formulas and the slightly different pairing functions.