Check code coverage

[eyalroz]

Now that we have (half-)decent unit test coverage (see #24), we should introduce code coverage checks to see how much remains uncovered.

This requires:

• Getting a coverage-related CMake module
• Changing CMakeLists.txt (both of them? only for tests?) to use that module and to generate reports after a successful build
• Working with a GitHub-interfacing service to visualize results or even check on the fly after each commit. Perhaps codecov?

See:

• https://stackoverflow.com/questions/24286349/cmake-ctest-code-coverage-check
• https://github.com/bilke/cmake-modules/blob/master/CodeCoverage.cmake
• https://github.com/codecov/example-cpp11-cmake
• https://codecov.io/
• https://docs.codecov.io/docs