Add code coverage for tests

[mfreeborn]

It would be really good to boost test coverage throughout the library, and one way to monitor that would be to use something like cargo-llvm-cov and codecov.io.

I've put together a small PR which adds the necessary steps to the two current workflow .yml files.

It would be necessary for this repo to add a 3rd party integration to codecov.io.

[mfreeborn]

@igiagkiozis if you approve, this would require you, as the owner, to briefly link the repo on codecov.io. Then I should be able to take it from there.