ConstPtr

Rusts std::ptr::NonNull wraps a *mut pointer. Sometimes this has implications that are not desired. ConstPtr wraps NullPtr but removes the mutable and uninit API's. As consequence it can only be constructed to point to a valid object (although it may become dangling when the object becomes destroyed). This simplifies handling lifetime erased references because a &reference implicitly coerces to a *const pointer and adds some safety net as in one can't accidentally get a mutable reference to something that should been const.