Restrict grammer of 2.0 macros

The arguments must be parenthesized and the body must be braced.
diff --git a/src/lib.rs b/src/lib.rs
index 106a045..edfea5c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -101,6 +101,8 @@
 #[cfg(feature = "parsing")]
 pub mod synom;
 pub mod delimited;
+#[cfg(feature = "parsing")]
+mod tt;
 
 mod gen {
     #[cfg(feature = "visit")]