Use range where possible in for loops

[phbnf]

Towards #230.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.53%. Comparing base (4ebea17) to head (8935a48).
⚠️ Report is 130 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/proofgen/main.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #231       +/-   ##
===========================================
- Coverage   89.33%   58.53%   -30.81%     
===========================================
  Files           7        8        +1     
  Lines         497      627      +130     
===========================================
- Hits          444      367       -77     
- Misses         48      253      +205     
- Partials        5        7        +2     

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