SMT2: implement cond

[kroening]

This adds support for cond_exprt, via a lowering to nested ?:, to the SMT backend.

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• 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).
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[kroening]

cond is also an SMV-only construct; it would likely be a candidate for removal once the solver architecture becomes pluggable.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Project coverage is 78.93%. Comparing base (fa299c5) to head (19958fe).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/std_expr.cpp	0.00%	12 Missing ⚠️
src/solvers/smt2/smt2_conv.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8467      +/-   ##
===========================================
+ Coverage    78.63%   78.93%   +0.29%     
===========================================
  Files         1728     1728              
  Lines       199408   198649     -759     
  Branches     18316    18355      +39     
===========================================
- Hits        156797   156795       -2     
+ Misses       42611    41854     -757     

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