Add deconstructor calls to tasks.
Our stack is not calling deconstructors of its elements. This is problematic for e.g. the graph implementation where reference counted images are hold in tasks. To solve this for now we manually call the deconstructor after each tasks (we do so, because a generic, virtual deconstructor adds runtime costs to primitive tasks, requiring us to re-run all benchmarks; with this change we do not need to do this and as we re-work the scheduler anyways we postpone a clean implementation for then).