do not create intermediate string for log level

src/lib.rs

@@ -20,7 +20,7 @@ impl Log for SimpleLogger {
             println!(
                 "{} {:<5} [{}] {}",
                 Local::now().format("%Y-%m-%d %H:%M:%S,%3f"),
-                record.level().to_string(),
+                record.level(),
                 record.module_path().unwrap_or_default(),
                 record.args());
         }