Resolve symbolic differentiation infinite loop under pypy

[jsiirola]

Fixes #3096 .

Summary/Motivation:

This resolves an infinite loop that was observed in symbolic differentiation when run under PyPy (#3096) by ensuring that the derivative expressions are detangled from the original expressions. As the walker is already creating the new lists, this change should have minimal impact on the walker's performance.

Changes proposed in this PR:

• Ensure derivative expressions are detangled from original expression
• Add test of the behavior observed in EXPR.differentiate() get stuck for a quadratic expression with mode=reverse_symbolic #3096.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (74f18cf) 88.25% compared to head (c7fe8fe) 88.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3098      +/-   ##
==========================================
+ Coverage   88.25%   88.26%   +0.01%     
==========================================
  Files         832      832              
  Lines       92390    92390              
==========================================
+ Hits        81539    81551      +12     
+ Misses      10851    10839      -12     

Flag	Coverage Δ
linux	85.99% <100.00%> (+0.01%)	⬆️
osx	75.54% <100.00%> (+<0.01%)	⬆️
other	86.17% <100.00%> (+<0.01%)	⬆️
win	83.40% <100.00%> (+<0.01%)	⬆️

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[michaelbynum]

Just for record keeping - the underlying issue here has to do with expression mutability/immutability. @jsiirola and I are exploring options.

[jsiirola]

This is being replaced by #3099