Begin experimental support for pin reborrowing

This commit adds basic support for reborrowing `Pin` types in argument
position. At the moment it only supports reborrowing `Pin<&mut T>` as
`Pin<&mut T>` by inserting a call to `Pin::as_mut()`, and only in
argument position (not as the receiver in a method call).

Author: eholk

core/src/pin.rs

@@ -1408,6 +1408,7 @@ impl<Ptr: DerefMut> Pin<Ptr> {
     ///     }
     /// }
     /// ```
+    #[cfg_attr(not(bootstrap), lang = "pin_as_mut")]
     #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
     pub fn as_mut(&mut self) -> Pin<&mut Ptr::Target> {