Some profile-guided optimization

[ice1000]

Recursive buildNested has made the profiler output harder to read:

After turning it into iteration, it's easier:

The sourcePosForSubExpr is taking a lot of time. Shit!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.59%. Comparing base (7c385f4) to head (0505c78).
Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1305      +/-   ##
============================================
+ Coverage     83.58%   83.59%   +0.01%     
- Complexity     4381     4382       +1     
============================================
  Files           396      396              
  Lines         12661    12664       +3     
  Branches       1451     1452       +1     
============================================
+ Hits          10583    10587       +4     
+ Misses         1304     1303       -1     
  Partials        774      774              

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