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.
* Initial implementation of quintic extensions.
* Update to/from_biguint() methods.
* Draft of fast multiplication on quintic extensions over 64-bit base.
* cargo fmt
* Typo.
* Document functions (a bit).
* Refactor reduction step.
* Change multiplication call so that LLVM generates better assembly.
* Use one main accumulator instead of two minor ones; faster reduce.
* Use one main accumulator in square too; clean up redundant code.
* Call faster routines from Mul and Square impls.
* Fix reduction function.
* Fix square calculation.
* Slightly faster reduction.
* Clean up names and types.
* cargo fmt
* Move extension field mul/sqr specialisations to their own file.
* Rename functions to have unique prefix.
* Add faster quadratic multiplication/squaring.
* Faster quartic multiplication and squaring.
* cargo fmt
* clippy
* Alternative reduce160 function.
* Typo.
* Remove alternative reduction function.
* Remove delayed reduction implementation of squaring.
* Enforce assumptions about extension generators.
* Make the accumulation variable a u32 instead of u64.
* Add test to trigger carry branch in reduce160.
* cargo fmt
* Some documentation.
* Clippy; improved comments.
* cargo fmt
* Remove redundant Square specialisations.
* Fix reduce*() visibility.
* Faster reduce160 from Jakub.
* Change mul-by-const functions to operate on 160 bits instead of 128.
* Move code for extensions of GoldilocksField to its own file.
* Split into crates
I kept other changes to a minimum, so 95% of this is just moving things. One complication that came up is that since `PrimeField` is now outside the plonky2 crate, these two impls now conflict:
```
impl<F: PrimeField> From<HashOut<F>> for Vec<u8> { ... }
impl<F: PrimeField> From<HashOut<F>> for Vec<F> { ... }
```
with this note:
```
note: upstream crates may add a new impl of trait `plonky2_field::field_types::PrimeField` for type `u8` in future versions
```
I worked around this by adding a `GenericHashOut` trait with methods like `to_bytes()` instead of overloading `From`/`Into`. Personally I prefer the explicitness anyway.
* Move out permutation network stuff also
* Fix imports
* Fix import
* Also move out insertion
* Comment
* fmt
* PR feedback
