CONTRACTS: represent loop nesting structure as a graph

[remi-delmas-3000]

Utility class for loop contracts instrumentation.

Co-authored with @qinheping

Extracted from #7541, can only be tested once all features are in place.

• 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.
• 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.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01 🎉

Comparison is base (b34e991) 78.50% compared to head (b4b59d9) 78.51%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7631      +/-   ##
===========================================
+ Coverage    78.50%   78.51%   +0.01%     
===========================================
  Files         1671     1674       +3     
  Lines       191847   191956     +109     
===========================================
+ Hits        150618   150723     +105     
- Misses       41229    41233       +4     

see 11 files with indirect coverage changes

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[tautschnig]

This now just needs fixes to Doxygen.