Encode union tag types in smt2 decision procedure

[thomasspriggs]

This is a first step towards union support and is only expected to be testable via unit test. I have raised this as a separate PR in an effort to keep the PRs for union support small and easy to review.

• 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 (e02fd74) 78.94% compared to head (bd798a1) 77.60%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7914      +/-   ##
===========================================
- Coverage    78.94%   77.60%   -1.35%     
===========================================
  Files         1697     1697              
  Lines       195379   195427      +48     
===========================================
- Hits        154242   151658    -2584     
- Misses       41137    43769    +2632     

Files	Coverage Δ
...vers/smt2_incremental/encoding/struct_encoding.cpp	96.26% <100.00%> (+1.16%)	⬆️
...vers/smt2_incremental/encoding/struct_encoding.cpp	100.00% <100.00%> (ø)

... and 70 files with indirect coverage changes

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