Heap size measurement

[wusyong]

First of all, thanks for building this crate! I found this crate is pretty great and GC types are ergonomic that I don't need to worry too much. The issue I encounter is I wish to get the heap size that GC controls to determine my context's workload. number_of_active_handles and number_of_tracked_allocations don't seem enough because there are various of types. Maybe we could define some traits to get the size of each GC objects like servo's malloc_size_of does?