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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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