Improve code coverage facilities #57

As per review comments in #55:

In the generated coverage.function.html, most "uncovered" functions are just template instantiations from unit tests/doc examples/benchmarks. The more instantiations there are, the less function coverage there will be. I think we should turn off function coverage if possible - it's meaningless for a template library.

We should investigate feasibility to integrate actions/upload-code-coverage