implement xnor in prop_conv_solvert

[kroening]

This adds xnor to prop_conv_solvert, mirroring the implementation for nor/nand.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.93%. Comparing base (436ed5d) to head (d877a12).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/prop/prop_conv_solver.cpp	71.42%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8533      +/-   ##
===========================================
- Coverage    78.93%   78.93%   -0.01%     
===========================================
  Files         1728     1728              
  Lines       198662   198664       +2     
  Branches     18346    18351       +5     
===========================================
  Hits        156811   156811              
- Misses       41851    41853       +2     

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