chore: remove addSubIterator and make iterators less mutable

[barakmich]

Description

Testing

References

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.60%. Comparing base (0b407f0) to head (7df045c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/query/build_tree.go	90.00%	1 Missing and 1 partial ⚠️

❌ Your project status has failed because the head coverage (74.60%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2898      +/-   ##
==========================================
- Coverage   74.60%   74.60%   -0.00%     
==========================================
  Files         484      484              
  Lines       58538    58531       -7     
==========================================
- Hits        43669    43663       -6     
+ Misses      11804    11802       -2     
- Partials     3065     3066       +1     

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[tstirrat15]

I dig