A trait for C++ subclasses of the superclass T. Implementing this trait
makes it possible for Rust code to be generic over all subclasses of a given
C++ supertype.
A trait for C++ types that have multiple different subclasses. Implementing
this for a superclass and Subclass for its subclasses makes it possible to
safely check for the object’s actual type based on its vtable.
Loads a static reference to T from an Rva that points directly to its
memory. Because this always assumes that the underlying object is
initialized, it can only return InstanceError::Null if rva itself is 0.