Increase code coverage

[adria0]

These changes should bump up coverage to 80%

• Use llvm-cov instead tarpaulin
• Disable coverage on graph & cost tools
• Move some examples to tests, those examples suits more into the test category than as examples
• Add unit test for Value. Also adds some #[must_use] that will help to catch errors.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.95%. Comparing base (e262eaf) to head (411064b).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #333       +/-   ##
===========================================
+ Coverage   55.25%   80.95%   +25.69%     
===========================================
  Files          92       80       -12     
  Lines        9635    16499     +6864     
===========================================
+ Hits         5324    13356     +8032     
+ Misses       4311     3143     -1168     

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[ed255]

LGTM!