Stablize pin feature (#814)

Box::pinned has been renamed to Box::pin. Meanwhile, the pin feature
no longer requires an attribute to enable.

Fixes: #813

tokio-async-await/src/compat/backward.rs

@@ -19,7 +19,7 @@ pub struct Compat<T>(Pin<Box<T>>);
 impl<T> Compat<T> {
     /// Create a new `Compat` backed by `future`.
     pub fn new(future: T) -> Compat<T> {
-        Compat(Box::pinned(future))
+        Compat(Box::pin(future))
     }
 }