Reference counting lets us pass these around a bit more easily.
SharedPtr was lightly tested using uint64_t in the main zion funcion.
Also add a sleep functionality instead of panicking. Functionally the
same right now since we don't preempt.
Removes the internal next ptr from the Thread class.
We avoid doing a memory operation when scheduling on yield because we
simply cycle the item to the back of the list.
Drew Galbraith