- Relax the restriction that roots must also be retainers, by changing
  the type of the 'r' argument to retainClosure from (StgClosure *) to
  retainer.  Now retainRoot can pass CCS_SYSTEM as the retainer for a
  root if the closure is itself not a retainer.

- Traverse roots from the stable ptr table, which might not also be
  retainers (hence the generalisation above).