Box:
  - support for type coercion, e.g. `Box<T>` to `Box<dyn U>` if T
    implements U

Vec:
  - implement new methods (prerequisites for nova-core and binder)
    - Vec::truncate()
    - Vec::resize()
    - Vec::clear()
    - Vec::pop()
    - Vec::push_within_capacity()
      - new error type: PushError
    - Vec::drain_all()
    - Vec::retain()
    - Vec::remove()
      - new error type: RemoveError
    - Vec::insert_within_capacity
      - new error type: InsertError
  - simplify Vec::push() using Vec::spare_capacity_mut()
  - split Vec::set_len() into Vec::inc_len() and Vec::dec_len()
    - add type invariant Vec::len() <= Vec::capacity
    - simplify Vec::truncate() using Vec::dec_len()