Implement popcount in flattening back-end

[tautschnig]

We can avoid lowering, and eventually re-use this as part of other algorithms.

• Each commit message has a non-empty body, explaining why the change was made.
• 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

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.48%. Comparing base (558ef2f) to head (c2d47e1).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/bv_utils.cpp	96.15%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8717   +/-   ##
========================================
  Coverage    80.48%   80.48%           
========================================
  Files         1697     1698    +1     
  Lines       208446   208477   +31     
  Branches        73       73           
========================================
+ Hits        167759   167789   +30     
- Misses       40687    40688    +1     

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