Use the STL versions of variant and optional in incremental smt2 decision procedure

[thomasspriggs]

This PR updates the incremental smt2 decision procedure to use the STL versions of variant and optional.

• 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 and coverable lines are covered by tests ✅

Comparison is base (6475b3c) 79.09% compared to head (062d6c6) 79.09%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8089      +/-   ##
===========================================
- Coverage    79.09%   79.09%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       196619   196617       -2     
===========================================
- Hits        155521   155519       -2     
  Misses       41098    41098              

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