Flattening backend: further cases for casts to range types

[kroening]

This adds further cases for casts to range types to the flattening backend (casts from bool, casts from signed bit vectors, casts to ranges that do not start at zero).

• 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

❌ Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (76dd20f) to head (93e6c3b).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/boolbv_typecast.cpp	0.00%	23 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8807      +/-   ##
===========================================
- Coverage    80.00%   80.00%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188249   188257       +8     
  Branches        73       73              
===========================================
+ Hits        150615   150618       +3     
- Misses       37634    37639       +5     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.