- Remove the unique objects used as slots since we decided 'null' was
  fine after all
- Rename 'slot' to 'newSlot' when it's used as an argument to change the
  _slot field, to clarify which variable has the newer value
- Remove the RenderObject registry since we'll do listeners a different
  way. This also removes handleEvent for the same reason.
- Remove the TODOs for mount/unmount becoming didMount/didUnmount since
  the methods do in fact do the mounting/unmounting.