* Some changes to generator_indices_by_watches
- Index generators by the representatives (in disjoint-set forest terminology) of their watched targets, rather than the watched targets themselves. Enqueuing generators based on their watch lists then works correctly, so we no longer need the step where we reenqueue all generators.
- In #195, it was pointed out that this slows down witness generation a bit. I moved the indexing code to preprocessing, so the prover is a bit faster (~7ms for me).
* Outdated comment
* Panic instead of infinite loop if we get stuck
* BTree
* fmt
Mainly storing pending generators in a Vec rather than a HashMap. Requires an extra check to make sure we don't run one twice after adding it to the Vec twice.
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).
