Drop does not clear up properly for BinaryHeap/HashMap/Stash

[cmichi]

I created/adapted Drop tests for some of our datastructures (implementation here).

Those fail though:

failures:
    collections::binary_heap::tests::drop_works
    collections::hashmap::tests::drop_works
    collections::stash::tests::drop_works

It seems the issue is that Pack does not implement Drop, which from my understanding can result in corrupted storage since e.g. for Stash its Drop implementation currently only clears Stash::elems: LazyIndexMap<T>, but not Stash::header: Pack<Header>.