* Simplify loop and remove clippy.
* Offset auxiliary coefficients so they're always positive.
* Split mul aux input into lo/hi parts.
* Rename register.
* Combine `QUO_INPUT_{LO,HI}`; rearrange some columns.
* Split `MODULAR_AUX_INPUT` into high and low pieces.
* Remove range_check_error debug output.
* First draft of generating the range checks.
* Remove opcodes for operations that were defined elsewhere.
* Clean up interface to build arithmetic trace.
* Fix "degree too high" bug in DIV by zero.
* Fix constraint_transition usage in recursive compare.
* Fix variable name; use named constant.
* Fix comment values.
* Fix bug in recursive MUL circuit.
* Superficial improvements; remove unnecessary genericity.
* Fix bug in recursive MULMOD circuit.
* Remove debugging noise; expand test.
* Minor comment.
* Enforce assumption in assert.
* Make DIV its own operation.
* Make MOD it's own operation; rename structs; refactor.
* Expand basic test.
* Remove comment.
* Put Stark operations in their own file.
* Test long traces.
* Minor comment.
* Address William's comments.
* Use `const_assert!` instead of `debug_assert!` because Clippy.
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.
