It seems redundant in most contexts, e.g. `use plonky2::field::extension_field::Extendable;`. One could import `extension_field`, but it's not that common in Rust, and `field::extension` is now about as short.
* First draft for division.
* `eval_division` work
* Division
* Minor: outdated fixme
* Tests and better column names
* Minor lints
* Remove redundant constraint
* Make division proof more formal
* Minor proof and comments
Co-authored-by: Hamish Ivey-Law <hamish@ivey-law.name>