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).
... and other minor refactoring.
`bench_recursion` will be the default bin run by `cargo run`; the otheres can be selected with the `--bin` flag.
We could probably delete some of the other binaries later. E.g. `field_search` might not be useful any more. `bench_fft` should maybe be converted to a benchmark (although there are some pros and cons, e.g. the bench framework has a minimum number of runs, and isn't helpful in testing multi-core performance).
