After extracting out `_top_for_allocations`, the users of `SpaceMangler` can better maintain `top`. This affects only Serial and Parallel. Other GCs already treat `SpaceMangler` as `AllStatic`, using its static API solely. `SpaceMangler` being stateful complicates the logic, which is probably the reason why a dev-flag `CheckZapUnusedHeapArea` is there to check correctness if needed. After the simplification, the dev-flag can also be removed.