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).
My recent change made `padded` panic if the padded length is less than the current length. I figured that might indicate that something unexpected was going on, so might be good to fail fast.
It looks like `inv_mod_xn` was relying on the old `padded` behavior, and it seems correct AFAIK, i.e. in this case it wasn't a symptom of anything going wrong.
We could also restore the old behavior of `padded` if you prefer; let me know if you have a preferennce.
