SMT2: implement cond

[kroening]

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

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• 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).
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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 78.37%. Comparing base (a209b44) to head (a47d672).

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    78.22%   78.37%   +0.14%     
===========================================
  Files         1726     1726              
  Lines       189038   188684     -354     
  Branches     18265    18286      +21     
===========================================
  Hits        147874   147874              
+ Misses       41164    40810     -354     

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