These appear to be unused for extension fields, so we're free to change the mapping without breaking anything.
As the TODO says, the mapping that's currently implemented doesn't seem natural or useful. It seems more natural to treat the `BigUint` as a base field element, potentially in a non-canonical form.
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.
