* Split up `PartitionWitness` data
This addresses two minor inefficiencies:
- Some preprocessed forest data was being cloned during proving.
- Some of the `ForestNode` data (like node sizes) is only needed in preprocessing, not proving. It was taking up cache space during proving because it was interleaved with data that is used during proving (parents, values).
Now `Forest` contains the disjoint-set forest. `PartitionWitness` is now mainly a Vec of target values; it also holds a reference to the (preprocessed) representative map.
On my laptop, this speeds up witness generation ~12%, resulting in an overall ~0.5% speedup.
* Feedback
* No size data (#278)
* No size data
* 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).
