Coverage: emit lossless lcov file

[AndrewNolte]

Branch info is supported by lcov, see here for explanations

We can modify the coverage annotation to fix confusing lcov data like https://github.com/verilator/verilator/issues/4691. The branch coverage view isn't great on the default lcov genhtml, but it can be interpreted by other viewers like gcovr, or a vscode extension