`Rc<Derived>` where an `Rc<Base>` is wanted

Decided

D-0064, settled by the language’s author: part of spec §18. Older text cites it as 18.64.

Converts implicitly, as &Derived → &Base does (18.32) and as C++’s shared_ptr<Derived> does: the same for Weak and Box. The count is shared, not copied. A handle to a trait object stays deferred, having no C++ counterpart to follow (§14.5)