Make Rc's representation look like Arc.

This lets me do some pretty horrible things with mem::transmute to
upgrade an Rc to an Arc in "safe" situations.

Yes it's horrible. But it's also really convenient!
Right, but `UnsafeCell` is actually a lang item, and the compiler is aware that any types containing `UnsafeCell` can do internal mutability. If there is no `UnsafeCell` then the compiler doesn't know that there can be mutation through an `&` reference and will optimize in ways that are incorrect.