Since the returned object is now owned by the callee object, we can
simply vend a ChildType&. This allows us to use "." instead of "->"
at the call site, which is quite nice. :^)
Consider the old pattern for creating a Core::Object parent and child:
auto parent = Core::Object::construct(...);
auto child = Core::Object::construct(..., parent);
The above was an artifact of the pre-reference-counting Object era.
Now that objects have less esoteric lifetime management, we can replace
the old pattern with something more expressive:
auto parent = Core::Object::construct(...);
auto child = parent->add<Core::Object>(...);
This reads a lot more naturally, and it also means we can get rid of
all the parent pointer arguments to Core::Object subclass constructors.
