With this change, we now have ~1200 CellAllocators across both LibJS and
LibWeb in a normal WebContent instance.
This gives us a minimum heap size of 4.7 MiB in the scenario where we
only have one cell allocated per type. Of course, in practice there will
be many more of each type, so the effective overhead is quite a bit
smaller than that in practice.
I left a few types unconverted to this mechanism because I got tired of
doing this. :^)
We should not GC allocate in the constructors of GC-allocated objects
because a new allocation might trigger garbage collection, which in
turn might access not fully initialized objects.
In FetchAlgorithms, it is common for callbacks to capture realms. This
can indirectly keep objects alive that hold FetchController with these
callbacks. This creates a cyclic dependency. However, when
JS::HeapFunction is used, this is not a problem, as captured by
callbacks values do not create new roots.
