* Working "faster" inverse algo, using u128s.
* Faster inverse_2exp for large exp.
* More inverse tests.
* Make f, g u64.
* Comments.
* Unroll first two iterations.
* Fix bug and re-unroll first two iterations.
* Simplify loop.
* Refactoring and documentation.
* Clean up testing.
* Move inverse code to inversion.rs; use in GoldilocksField.
* Bench quartic Goldilocks extension too.
* cargo fmt
* Add more documentation.
* Address Jakub's comments.
- 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.
