Function export_step_with_log_to_writer 

pub fn export_step_with_log_to_writer<W: Write>(
    content: &[u8],
    opts: &StepOptions,
    log: &MutationLog,
    w: &mut W,
) -> Result<LogExportStats>

export_step_with_log, writing as it goes. The source is read, never copied: memory beyond the record index grows with the EDITS (the lines they change, the records they generate), not with the file.