Implement fmt::Pointer for pointers to unsized types

This allows printing pointers to unsized types with the {:p} formatting
directive. The following impls are extended to unsized types:
 - impl<'a, T: ?Sized> Pointer for &'a T
 - impl<'a, T: ?Sized> Pointer for &'a mut T
 - impl<T: ?Sized> Pointer for *const T
 - impl<T: ?Sized> Pointer for *mut T
 - impl<T: ?Sized> fmt::Pointer for Box<T>
 - impl<T: ?Sized> fmt::Pointer for Rc<T>
 - impl<T: ?Sized> fmt::Pointer for Arc<T>

src/liballoc/arc.rs

@@ -879,7 +879,7 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> fmt::Pointer for Arc<T> {
+impl<T: ?Sized> fmt::Pointer for Arc<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Pointer::fmt(&*self._ptr, f)
     }

src/liballoc/boxed.rs

@@ -442,7 +442,7 @@ impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> fmt::Pointer for Box<T> {
+impl<T: ?Sized> fmt::Pointer for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         // It's not possible to extract the inner Uniq directly from the Box,
         // instead we cast it to a *const which aliases the Unique

src/liballoc/rc.rs

@@ -688,7 +688,7 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> fmt::Pointer for Rc<T> {
+impl<T: ?Sized> fmt::Pointer for Rc<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Pointer::fmt(&*self._ptr, f)
     }