Add opaque type attributes

This allows for the code to compile on `nightly`.

src/opaque-types-impl-trait-inference.md

@@ -13,13 +13,16 @@ it can work across functions and function bodies.
 To help explain how it works, let's consider an example.
 
 ```rust
+#![feature(type_alias_impl_trait)]
 mod m {
     pub type Seq<T> = impl IntoIterator<Item = T>;
 
+    #[define_opaque(Seq)]
     pub fn produce_singleton<T>(t: T) -> Seq<T> {
         vec![t]
     }
 
+    #[define_opaque(Seq)]
     pub fn produce_doubleton<T>(t: T, u: T) -> Seq<T> {
         vec![t, u]
     }