`Shared<T>::deref()` should be marked as unsafe

[jeehoonkang]

Related discussion: https://github.com/stjepang/coco/pull/2

The dereference of a Shared<T>, may race with the writer of the shared data of type T. For example, suppose a Box<T> is written to Atomic<T> with store_ref(Relaxed), and later another thread read the Atomic<T> with load(Relaxed). Then the Box<T> itself may not have been synchronized, and the dereference of the loaded Shared<T> may invoke undefined behavior.