* First draft of 256-bit addition.
* Update comment.
* cargo fmt
* Rename addition evaluation file.
* Port ALU logic from SZ.
* Give a name to some magic numbers.
* `addition.rs` -> `add.rs`; fix carry propagation in add; impl sub.
* Clippy.
* Combine hi and lo parts of the output.
* Implement MUL.
* Suppress Clippy's attempt to make my code even harder to read.
* Next draft of MUL.
* Make all limbs (i.e. input and output) 16-bits.
* Tidying.
* Use iterators instead of building arrays.
* Documentation.
* Clippy is wrong; also cargo fmt.
* Un-refactor equality checking, since it was wrong for sub.
* Daniel comments.
* Daniel comments.
* Rename folder 'alu' -> 'arithmetic'.
* Rename file.
* Finish changing name ALU -> Arithmetic Unit.
* Finish removing dependency on array_zip feature.
* Remove operations that will be handled elsewhere.
* Rename var; tidy up.
* Clean up columns; mark places where range-checks need to be done.
* Import all names in 'columns' to reduce verbiage.
* cargo fmt
* Fix aux_in calculation in mul.
* Remove redundant 'allow's; more precise range-check size.
* Document functions.
* Document MUL instruction verification technique.
* Initial tests for ADD.
* Minor test fixes; add test for SUB.
* Fix bugs in generate functions.
* Fix SUB verification; refactor equality verification.
* cargo fmt
* Add test for MUL and fix some bugs.
* Update doc.
* Quiet incorrect clippy error.
* Clean up 'decode.rs'.
* Fold 'decode.rs' into 'arithmetic_stark.rs'.
* Force limb size to divide EVM register size.
* Document range-check warning and fix end value calc.
* Convert `debug_assert!`s into `assert!`s.
* Clean up various kinds of iterator usage.
* Remove unnecessary type spec.
* Document unexpected use of `collect`.
