Add support for union member expressions in incremental smt2 decision procedure

[thomasspriggs]

This PR adds support for union member expressions in incremental smt2 decision procedure. This support is sufficient for some of the existing union regression tests to pass.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (a531056) 78.75% compared to head (5985e55) 78.83%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7961      +/-   ##
===========================================
+ Coverage    78.75%   78.83%   +0.08%     
===========================================
  Files         1701     1701              
  Lines       196224   196246      +22     
===========================================
+ Hits        154540   154719     +179     
+ Misses       41684    41527     -157     

Files	Coverage Δ
...vers/smt2_incremental/encoding/struct_encoding.cpp	96.85% <100.00%> (+0.07%)	⬆️
...vers/smt2_incremental/encoding/struct_encoding.cpp	100.00% <100.00%> (ø)

... and 40 files with indirect coverage changes

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