Tests for Expression and ModifierExpression + some performance stuff

[gausie]

• Add tests for Expression and ModifierExpression
• Fix some things that are broken (no one noticed that path() just didn't work, for example)
• Avoid string concatenation in loops in expression
• Do some simple performance improvements (like using isBlank() instead of comparing to a blank string)
• Remove some unnecessary iteration by using addAll etc

[codecov]

Codecov Report

Merging #540 (e2814c7) into main (7c4de0e) will increase coverage by 0.15%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##               main     #540      +/-   ##
============================================
+ Coverage     19.60%   19.76%   +0.15%     
- Complexity     8095     8184      +89     
============================================
  Files           998      998              
  Lines        155923   155938      +15     
  Branches      34681    34670      -11     
============================================
+ Hits          30566    30817     +251     
+ Misses       119664   119454     -210     
+ Partials       5693     5667      -26     

Impacted Files	Coverage Δ
src/net/sourceforge/kolmafia/AreaCombatData.java	14.80% <0.00%> (-0.19%)	⬇️
src/net/sourceforge/kolmafia/CakeArenaManager.java	0.00% <0.00%> (ø)
src/net/sourceforge/kolmafia/FamiliarData.java	22.64% <0.00%> (ø)
src/net/sourceforge/kolmafia/KoLAdventure.java	15.18% <0.00%> (-0.24%)	⬇️
src/net/sourceforge/kolmafia/KoLmafia.java	8.07% <0.00%> (ø)
src/net/sourceforge/kolmafia/KoLmafiaASH.java	12.50% <0.00%> (ø)
src/net/sourceforge/kolmafia/KoLmafiaCLI.java	67.95% <0.00%> (ø)
src/net/sourceforge/kolmafia/KoLmafiaGUI.java	0.00% <0.00%> (ø)
src/net/sourceforge/kolmafia/PastaThrallData.java	50.00% <0.00%> (+8.82%)	⬆️
src/net/sourceforge/kolmafia/RequestEditorKit.java	17.93% <0.00%> (ø)
... and 42 more

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