This ensures that we are explicitly declaring the allocator to use when
allocating a cell(-inheriting) type, instead of silently falling back
to size-based allocation.
Since this is done in allocate_cell, this will only be detected for
types that are actively being allocated. However, since that means
they're _not_ being allocated, that means it's safe to not declare
an allocator to use for those. For example, the base TypedArray<T>,
which is never directly allocated and only the defined specializations
are ever allocated.
HeapVector inherits from GC::Cell, and thus participates in tracing
garbage collection. It's not a standalone vector of roots like
RootVector or ConservativeVector. It must be marked for its elements to
be marked.
