Unify `BigInteger*` and `Fp*` types once `min_const_generics` is stable

[Pratyush]

min_const_generics will be stable soon: rust-lang/rust#79135

This is an opportunity to unify all the BigInteger structs, and also all the Fp structs (which can maybe become Fp<B: BigInteger>?). This should help with compile times, and with duplication of code, and should allow us to remove many macros, thus potentially resolving #3.