Add code coverage

[marcoieni]

In #43 I removed the current code coverage approach because it wasn't working anymore.
What's the best way to do code coverage in rust today?

Possibilities:

• https://github.com/taiki-e/cargo-llvm-cov (example here)

[ghenry]

Solved in that PR?

[marcoieni]

potentially I think, but the PR still needs work