Add case_exprt to std_expr.h and refactor code to use it

[tautschnig]

Introduces case_exprt for improved type safety.

Fixes: #3037

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[codecov]

Codecov Report

❌ Patch coverage is 91.20879% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (4fe3ade) to head (18d5083).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/std_expr.h	86.11%	5 Missing ⚠️
src/solvers/flattening/boolbv.cpp	0.00%	2 Missing ⚠️
src/solvers/flattening/boolbv_case.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8741   +/-   ##
========================================
  Coverage    79.93%   79.94%           
========================================
  Files         1698     1699    +1     
  Lines       187698   187785   +87     
  Branches        73       73           
========================================
+ Hits        150034   150120   +86     
- Misses       37664    37665    +1     

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