use power_exprt in boolbvt

[kroening]

This replaces the use of binary_exprt in boolbvt by the more specific power_exprt.

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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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
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[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (af0b511) to head (d552f91).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/boolbv_power.cpp	0.00%	6 Missing ⚠️
src/solvers/flattening/boolbv.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8833      +/-   ##
===========================================
- Coverage    80.00%   80.00%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188248   188252       +4     
  Branches        73       73              
===========================================
  Hits        150613   150613              
- Misses       37635    37639       +4     

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