Inject metadata into the llvm-cov JSON output

[dnaka91]

As discussed in #285, this injects some metadata into the JSON output of llvm-cov. This can be helpful for tools that rely on the output of it and cargo-llvm-cov as well.

It adds in the following fields under a new cargo_llvm_cov field in the root JSON structure:

• version is the cargo-llvm-cov version that was invoked. Helpful for tools to verify that cargo-llvm-cov is of a certain version and has some expected behavior.
• manifest_path is the location of the Cargo.toml as resolved by cargo-llvm-cov. This is two-fold:
  • It can reduce repetition, as otherwise a custom manifest path would need to specified for both cargo-llvm-cov and the other tool.
  • There might be minor differences in how the mainfest path was resolved, so this ensures the other tool gets exactly the same manifest path as cargo-llvm-cov.

Fixes #285

[taiki-e]

LGTM. Could you resolve the merge conflict?

[dnaka91]

Done. Just ping me in case any conflict comes up again :bowing_man:

[taiki-e]

Thanks! Published in 0.5.23.