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.
* Halo2 style lookup arguments in System Zero
It's a really nice and simple protocol, particularly for the verifier since the constraints are trivial (aside from the underlying batched permutation checks, which we already support). See the [Halo2 book](https://zcash.github.io/halo2/design/proving-system/lookup.html) and this [talk](https://www.youtube.com/watch?v=YlTt12s7vGE&t=5237s) by @daira.
Previously we generated the whole trace in row-wise form, but it's much more efficient to generate these "permuted" columns column-wise. So I changed our STARK framework to accept the trace in column-wise form. STARK impls now have the flexibility to do some generation row-wise and some column-wise (without extra costs; there's a single transpose as before).
* sorting
* fixes
* PR feedback
* into_iter
* timing
* Batch multiple perm args into one Z and compute Z columnwise
It's slightly complex because we batch `constraint_degree - 1` permutation arguments into a single `Z` polynomial. This is a slight generalization of the [technique](https://zcash.github.io/halo2/design/proving-system/lookup.html) described in the Halo2 book.
Without this batching, we would simply have `num_challenges` random challenges (betas and gammas). With this batching, however, we need to use different randomness for each permutation argument within the same batch. Hence we end up generating `batch_size * num_challenges` challenges for all permutation arguments.
* Feedback + updates for recursion code
* Column definitions for addition, range checks & lookups
I implemented addition (unsigned for now) as an example of how the arithmetic unit can interact with the 16-bit range check unit.
Range checks and lookups aren't implemented yet.
* Missing constraints
* Tweaks to get tests passing
* Reorg registers into files
* Minor
