chore: DRY out expression parser

[danvk]

Factor out the commonalities between operators at different precedence levels. I had to reorder some of the rules (in code) to avoid forward references. I think this is nicer, but we'll see how this holds up!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e0e164c) 87.32% compared to head (0946296) 86.76%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
- Coverage   87.32%   86.76%   -0.56%     
==========================================
  Files           8        8              
  Lines         568      544      -24     
  Branches       74       69       -5     
==========================================
- Hits          496      472      -24     
  Misses         71       71              
  Partials        1        1              

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