Smart pointers

Smart pointers encode ownership strategies in types. This theme covers Box<T> for heap allocation and recursive types, Rc<T> for shared single-thread ownership, and Arc<T> for shared thread-safe ownership. Planned exercises: 1. choose Box for recursive data. 2. recognise shared ownership with Rc. 3. choose Arc across threads. 4. fill Box::new. 5. fill Rc::clone. 6. write a boxed enum node. 7. write shared ownership without cloning the inner value. 8. translate TS shared object references. 9. fix a type whose size would otherwise be infinite.