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.
* Complete versions of rotate left and shift left.
* Implement rotate/shift right.
* cargo fmt
* Fix documentation.
* Reduce visibility of helper functions.
* Address Jaqui's PR comments.
* Disable fall-through check as the run test assumes no failure here.
* Try to fix doctest failure.
