- Split it into two files, one for general `Field` tests and one for `PrimeField` tests.
- Replace most uses of `BigUint` in tests with `u64`. These uses were only applicable for `PrimeField`s, which are 64-bit fields anyway. This lets us delete the `BigUInt` conversion methods.
- Simplify `test_inputs`, which was originally written for large prime fields. Now that it's only used for 64-bit fields, I think interesting inputs are just the smallest and largest elements, and those close to 2^32 etc.
