Although we already have an option in CMake which add the needed compiler flags in GCC and CLANG to produce coverage information (this is used by the tools we employ in the PRs to generate coverage reports) it would be nice to also add some targets in the CMake configuration to generate local coverage reports.

This would be useful for exploring not exercised paths in the code while testing eit

We used to have this (tracked in felixc/rexiv2#18, set up in felixc/rexiv2@90ae678), but it had to be removed in felixc/rexiv2@1aff63e because the travis-cargo project we'd been using had broken ages ago was no longer actually uploading coverage info.