Function instrument 

pub fn instrument(
    source: &str,
    path: &str,
    source_type: SourceKind,
    options: &MutationOptions,
) -> Result<Instrumented, MutationError>

Finds the mutants in source and returns the instrumented text.

path is part of each mutant id. Use the project-relative path, so that ids are the same on each machine.