Better error handling in REPL

We have much better spanned expression support, so now we get to show
it off!

src/repl.rs

@@ -15,13 +15,12 @@ pub fn repl() {
                     break;
                 }
                 let mut parser = Parser::new(&line);
-                let ast = parser.init();
+                let value = parser.init().and_then(|ast| {
-                match ast {
+                    println!("{:?}", &ast);
-                    Ok(ast) => {
+                    env.eval_stmts(&ast)
-                        println!("{:?}", ast);
+                });
-                        println!("{:?}", env.eval_stmts(&ast));
+
-                    }
+                if let Err(e) = value {
-                    Err(e) => {
                     println!(
                         "Error `{:?}` occurred at span: {:?} = `{:?}`",
                         e.kind,
@@ -37,7 +36,6 @@ pub fn repl() {
                     );
                 }
             }
-            }
             Err(rustyline::error::ReadlineError::Eof) => {
                 println!("bye");
                 break;