Add the DerefImm and DerefMut traits.
Implement DerefImm for Rc and DerefImm/DerefMut for RefCell's Ref/RefMut.
Allow overloading explicit dereferences.
auto merge of #12491 : eddyb/rust/deref, r=nikomatsakis

Add the `Deref` and `DerefMut` traits and implement overloading explicit dereferences.