This is a weak pointer that integrates with the garbage collector.
It has a number of differences compared to AK::WeakPtr, including:
- The "control block" is allocated from a well-packed WeakBlock owned by
the GC heap, not just a generic malloc allocation.
- Pointers to dead cells are nulled out by the garbage collector
immediately before running destructors.
- It works on any GC::Cell derived type, meaning you don't have to
inherit from AK::Weakable for the ability to be weakly referenced.
- The Weak always points to a control block, even when "null" (it then
points to a null WeakImpl), which means one less null check when
chasing pointers.
