Refactor assigns clause implementation

[SaswatPadhi]

I was planning to merge these changes as part of #6249, but given the volume of changes, I thought it would be best to create a separate PR.

Main changes are:

• bug fixes in containment (previously called alias) and subset (previously called subset) checks
• usused fields (e.g., function_id, init_block) are removed,
• redundant functions (e.g. pointer_for) are removed,
• const references are used throughout consistently
• targets are tracked in an unordered_set as opposed to vector
• introduced a "normalize" method for extracting base object of arrays

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[SaswatPadhi]

Marked draft for now.

assigns_enforce_16 is failing but I cannot reproduce the failure locally!

[SaswatPadhi]

Marked draft for now.

assigns_enforce_16 is failing but I cannot reproduce the failure locally!

Fixed this by removing dead variables from freely assignable set all together (although the implementation is still incorrect and, as discussed offline, @feliperodri would make a separate PR to fix that properly).

But, what's concerning here is that goto-cc / goto-instrument was generating different goto binaries on my machine vs CI.

EDIT: The remaining CI issues with Windows tests seem completely unrelated.

[jimgrundy]

I realize I'm late here and that the coding practices for this code base are established ... but I would want to see more comments in the code. While particularly valuable for a new comer like myself, a header comment per method stating what it does, and maybe also per method argument/result, would help a great deal. I imagine one day being able to run doxygen on the code and learning something useful from it.

[codecov]

Codecov Report

Merging #6334 (e1e7235) into develop (aeba3f0) will decrease coverage by 0.00%.
The diff coverage is 96.20%.

@@             Coverage Diff             @@
##           develop    #6334      +/-   ##
===========================================
- Coverage    75.89%   75.89%   -0.01%     
===========================================
  Files         1515     1515              
  Lines       163972   163969       -3     
===========================================
- Hits        124444   124440       -4     
- Misses       39528    39529       +1     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.cpp	92.37% <85.71%> (+0.04%)	⬆️
src/goto-instrument/contracts/assigns.cpp	97.40% <98.30%> (-0.38%)	⬇️
src/goto-instrument/contracts/assigns.h	100.00% <100.00%> (ø)
src/goto-programs/goto_program.h	89.30% <0.00%> (-0.20%)	⬇️

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[SaswatPadhi]

Will push a few more things before final review & merge, so marked as draft.

[feliperodri]

Thank you for all the subsequent clean up and fixes!