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