KCov code coverage collection

[azriel91]

Implement code coverage collection using KCov.

Test binaries to be run can be discovered through this command (source: rust-lang/cargo#1924 (comment)):

$ cargo test --no-run --message-format=json | jq -r "select(.profile.test == true) | .filenames[]"

We may be able to collect code coverage for crate binaries by using that --message-format=json method to get the path to the binary and wrapping it with kcov in integration tests.

For a JSON query library, jmespath is the most feature complete right now, jsonpath is not yet feature complete, but looks promising (check the state of that crate when you actually do this ticket!).