format_expr now prints bv-typed constants

[kroening]

This adds formatting for bv-type constants to format_expr. As the bv type does not have a numerical representation, the 0/1 bit pattern is printed, starting with the bit with the highest index.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.39%. Comparing base (5002f3b) to head (2ed72b0).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8457      +/-   ##
===========================================
+ Coverage    78.35%   78.39%   +0.03%     
===========================================
  Files         1726     1726              
  Lines       188695   188624      -71     
  Branches     18301    18265      -36     
===========================================
+ Hits        147851   147864      +13     
+ Misses       40844    40760      -84     

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