* Use built-in `reverse_bits`; remove duplicate `reverse_index_bits`.
* Reduce precomputation time/space complexity from quadratic to linear.
* Several working cache-friendly FFTs.
* Fix to allow FFT of constant polynomial.
* Simplify FFT strategy choice.
* Add PrimeField and CHARACTERISTIC properties to Fields.
* Add faster method for inverse of 2^m.
* Pre-compute some of the roots; tidy up loop iteration.
* Precomputation for both FFT variants.
* Refactor precomputation; add optional parameters; rename some things.
* Unrolled version with zero tail.
* Iterative version of Unrolled precomputation.
* Test zero tail algo.
* Restore default degree.
* Address comments from @dlubarov and @wborgeaud.
