simplify: rewrite bitxnor on booleans to equal

[kroening]

This adds the rewrite a xnor b --> a = b for booleans a, b to the simplifier.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.53%. Comparing base (0fc8a49) to head (5d673ef).

Files with missing lines	Patch %	Lines
src/util/simplify_expr_int.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8594      +/-   ##
===========================================
- Coverage    79.63%   79.53%   -0.10%     
===========================================
  Files         1733     1733              
  Lines       197686   197929     +243     
  Branches     18226    18170      -56     
===========================================
  Hits        157418   157418              
- Misses       40268    40511     +243     

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[tautschnig]

Below we call simplify_boolean on the resulting expression, which won't know how to simplify equality. We'll have to change the call below to simplify_node.