Add CodeCov

[brunoocasali]

⚠️ This issue is generated, it means the examples and the namings do not necessarily correspond to the language of this repository.
Also, if you are a maintainer, please add any clarification and instructions about this issue.

Sorry if this is already wholly/partially implemented. Feel free to let me know about the state of this issue in the repo.

Related to meilisearch/integration-guides#59

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Codecov helps us to know if a PR add non-tested code. If it's the case, we don't accept the PR.
It's easy to install, easy to configure.

TODO:

• Add CodeCov to this repository.
• Add the CodeCov badge on this repository README.md
• Ensure a minimum coverage is accepted (optional)

[irevoire]

Hey @brunoocasali, I don’t know much about codecov, but in rust when people want to do codecoverage, they 100% use this lib: https://github.com/xd009642/tarpaulin

[carlosb1]

I have some experience with this tool..Also, check this: https://github.com/taiki-e/cargo-llvm-cov

Working with tarpaulin, I found some weird not coveraged cases that we never understood it...