core: Add stability attributes to Clone

The following are tagged 'unstable'

- core::clone
- Clone
- Clone::clone
- impl Clone for Arc
- impl Clone for arc::Weak
- impl Clone for Rc
- impl Clone for rc::Weak
- impl Clone for Vec
- impl Clone for Cell
- impl Clone for RefCell
- impl Clone for small tuples

The following are tagged 'experimental'

- Clone::clone_from - may not provide enough utility
- impls for various extern "Rust" fns - may not handle lifetimes correctly

See https://github.com/rust-lang/rust/wiki/Meeting-API-review-2014-06-23#clone

src/liballoc/arc.rs

@@ -110,6 +110,7 @@ impl<T: Share + Send> Arc<T> {
     }
 }
 
+#[unstable]
 impl<T: Share + Send> Clone for Arc<T> {
     /// Duplicate an atomically reference counted wrapper.
     ///
@@ -236,6 +237,7 @@ impl<T: Share + Send> Weak<T> {
     }
 }
 
+#[unstable]
 impl<T: Share + Send> Clone for Weak<T> {
     #[inline]
     fn clone(&self) -> Weak<T> {