Format bit-vectors with [ ... ] vector notation

[kroening]

To avoid confusion between bit-vectors and decimal numbers, format bv-typed bit-vectors using [ ... ] vector notation.

• 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 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.84%. Comparing base (57be2aa) to head (3b9e94e).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/format_expr.cpp	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8514      +/-   ##
===========================================
+ Coverage    78.51%   78.84%   +0.32%     
===========================================
  Files         1728     1728              
  Lines       199553   198775     -778     
  Branches     18303    18406     +103     
===========================================
+ Hits        156687   156719      +32     
+ Misses       42866    42056     -810     

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