Report source-based coverage results

[adpaco-aws]

Requested feature: The approach introduced in #2612 is a good start for coverage, but the end goal is to have source-based coverage results which are clearer than line-based for users. Maybe this way we can avoid #2639 while at the same time improving how report coverage.
Use case: Coverage results
Link to relevant documentation (Rust reference, Nomicon, RFC): #2612