Switch to using tracing instead of logging.

src/main.rs

@@ -5,7 +5,8 @@ mod parser;
 const TEST_DOC: &'static str = include_str!("../toy_language.txt");
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    pretty_env_logger::init();
+    let subscriber = tracing_subscriber::FmtSubscriber::new();
+    tracing::subscriber::set_global_default(subscriber)?;
     println!("{}\n\n\n", TEST_DOC);
     println!("{:#?}", document(TEST_DOC));
     Ok(())