SMT2: implement nand, nor, xnor

[kroening]

This adds the multi-ary Boolean operators nand and nor to the SMT2 backend.

• 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 5.55556% with 17 lines in your changes missing coverage. Please review.

Project coverage is 78.93%. Comparing base (fa299c5) to head (10eac1d).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	5.55%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8530      +/-   ##
===========================================
+ Coverage    78.63%   78.93%   +0.29%     
===========================================
  Files         1728     1728              
  Lines       199408   198653     -755     
  Branches     18316    18332      +16     
===========================================
+ Hits        156797   156798       +1     
+ Misses       42611    41855     -756     

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