Implement incremental SMT2 support for bswap_exprt

[tautschnig]

Implements byte swapping in the incremental SMT2 decision procedure and adds unit tests. Regression test gcc_bswap1 now passes.

Fixes: #8064

• 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.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (4fe3ade) to head (d5ab4af).

Files with missing lines	Patch %	Lines
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	91.30%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8777      +/-   ##
===========================================
+ Coverage    79.93%   79.94%   +0.01%     
===========================================
  Files         1698     1698              
  Lines       187698   187775      +77     
  Branches        73       73              
===========================================
+ Hits        150034   150117      +83     
+ Misses       37664    37658       -6     

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