The three friend classes below in `ZAllocator` are unnecessary. And actually, the `ZAllocatorSurvivor` and `ZAllocatorOld` don't exist.
```
// ZAllocator.hpp
friend class ZAllocatorEden;
friend class ZAllocatorSurvivor;
friend class ZAllocatorOld;
```
```
// ZAllocator.hpp
friend class ZAllocatorEden;
friend class ZAllocatorSurvivor;
friend class ZAllocatorOld;
```