* Add a tree of scopes for proving times
To replace the old `timed!` macro, which had no hierarchy.
It's similar to `ContextTree`, which tracks gate counts of circuit operations.
This gives a more organized output, with indentation levels based on scope depth, parent durations listed before child durations, etc.
* PR feedback
No functional changes here. The biggest change was moving certain files into new directories like `plonk` and `iop` (for things like `Challenger` that could be used in STARKs or other IOPs). I also split a few files, renames, etc, but again nothing functional, so I don't think a careful review is necessary (just a sanity check).
- Configure FRI with a list of arities that's more appropriate for a 2^14 instance. The previous config resulted in a huge final polynomial.
- Log the blinding factors, and other logging tweaks.
* 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.
