Define a struct that wraps some field-dependent type in an enum. Since Fp<P2> through Fp<P7>
are all ZSTs, working with them under the hood is in theory much faster than working with an
arbitrary prime field, let alone finite field.
Use all primes. It isnβt possible to have this import when odd-primes is disabled because
primes other than P2 and ValidPrime (which happen to be the same type) do not exist.