A storage policy for use with _Pointer_adapter<> which yields a standard pointer.
A _Storage_policy is required to provide 4 things: 1) A get() API for returning the stored pointer value. 2) An set() API for storing a pointer value. 3) An element_type typedef to define the type this points to. 4) An operator<() to support pointer comparison. 5) An operator==() to support pointer comparison.