Fix part of #4044: Add protos & testing library for commutative expressions/operations

[BenHenning]

Explanation

Fix part of #4044
Originally copied from #2173 when it was in proof-of-concept form

TODO: complete

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
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For UI-specific PRs only

N/A -- proto & testing library only change, and the proto changes are only additions. No UI functionality is yet affected by these changes.