ConstPtr

Rusts std::ptr::NonNull is a mutable 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 be const.

no_std compatibility

By default the 'std' feature is enabled which pulls in dependencies on rusts stdlib. When one disables this in 'default' then ConstPtr becomes no-std compatible.