Refactor - `LoadedPrograms::assign_program()`

[Lichtso]

Problem

Summary of Changes

• Forbids all program replacements except for reloads and builtins.
• Removes test_assign_program_tombstones()
• Adds test_assign_program_failure() and test_assign_program_success().

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.59259% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 81.6%. Comparing base (537c3d8) to head (89b434d). Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #35233     +/-   ##
=========================================
- Coverage    81.6%    81.6%   -0.1%     
=========================================
  Files         834      834             
  Lines      224798   224829     +31     
=========================================
+ Hits       183485   183499     +14     
- Misses      41313    41330     +17     

[pgarg66]

LGTM. @alessandrod, are you planning to review as well?

[alessandrod]

Yep going to review today