Tests have no notion of coverage

[blt]

Right now cernan tests do not make coverage reports. This makes it difficult to determine how well tested cernan is in practice. Issues that have snuck by into the wild would suggest that it is not.

[blt]

Current rub: gcov/kcov--which are the main tools for coverage detection of rust programs--are Linux only. There is some discussion around using LLVM's native coverage, but no in-progress work: rust-lang/rust#34701.

[tsantero]

As discussed, i'll add a Dockerfile to this project so that we can achieve coverage locally on macs.

[blt]

We can resolve this by integrating with codecov.io when open-sourcing Cernan.

[blt]

Now resolved! :D