Set up code coverage

[Morwenn]

codecov.io doesn't seem too hard to use thanks to GitHub extensions.

The idea would be to have a dedicated build with -O0 to make code coverage easier. I don't think that testing every path for every compiler or OS is worth it, I'll wait for some time to see which builds are the fastest now that we switch from Catch to Catch2: it used to be g++ builds but now clang++ builds seem to be faster. Not sure whether it has something to do with the cache, so I'll wait a bit more before deciding.

[Morwenn]

g++ on Linux seems to be the fastest when the cache is hot, so that will be the target for the coverage branch.

EDIT: apparently it's due to ccache not working properly with Clang without workarounds. I will have to handle that first.

[Morwenn]

Fixed by 74f4a7b