Enforce expression immutability in expr.args

[jsiirola]

Fixes #3096, superceeds #3098

Summary/Motivation:

This resolves #3096 by closing a loophole in the "immutability" of expressions. For efficiency, SumExpression makes use of a mutable data structure (list) for storing the node arguments. Prior to this PR, we would return the raw underlying argument list from SumExpression.args. There are, however, edge cases where modifying either the expression or an entangled expression can cause that list to change. This PR changes the SumExpression API so that SumExpression.args always makes a copy of the _args_ list, thereby ensuring that the interface preserves the immutability that users expect.

Note that this PR has a negative effect on performance in the form of a ~1% slow-down on LP and NL writers (as observed in the performance test suite).

Changes proposed in this PR:

• SumExpression.args now always returns a copy of the underlying argument list
• Add a test for EXPR.differentiate() get stuck for a quadratic expression with mode=reverse_symbolic #3096
• Clean up some of the expression API (remove redundant code)

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[michaelbynum]

Thank you for this @jsiirola!!! Looks great. I'm about to take a look at the failing test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (85b63ab) 88.26% compared to head (0d00410) 88.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3099      +/-   ##
==========================================
- Coverage   88.26%   88.26%   -0.01%     
==========================================
  Files         832      832              
  Lines       92385    92345      -40     
==========================================
- Hits        81544    81504      -40     
  Misses      10841    10841              

Flag	Coverage Δ
linux	86.01% <100.00%> (-0.01%)	⬇️
osx	75.53% <100.00%> (-0.02%)	⬇️
other	86.19% <100.00%> (-0.01%)	⬇️
win	83.42% <100.00%> (-0.01%)	⬇️

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