implement flattening for +/- for the range type

[kroening]

The flattening solver backend lowers the integer range type to bitvectors. This adds support for addition and subtraction for the range type via lowering to bitvector addition/subtraction.

• 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.
• 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 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.02%. Comparing base (bf58af8) to head (1a020c4).

Files	Patch %	Lines
src/solvers/flattening/boolbv_add_sub.cpp	16.66%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8396      +/-   ##
===========================================
- Coverage    78.02%   78.02%   -0.01%     
===========================================
  Files         1726     1726              
  Lines       189432   189442      +10     
  Branches     18465    18466       +1     
===========================================
+ Hits        147807   147808       +1     
- Misses       41625    41634       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.