Use-after-free when items panic in drop impls

[jdygert-spok]

Deque::clear, HistoryBuf::clear, and IndexMap::clear will each invalidate the invariants of those types if an item's drop impl panics. This can lead to use-after-drop in safe code using catch_unwind. These methods should reset the relevant fields before dropping the items.

[sgued]

Indeed this is a bug.

We cannot use catch_unwind because the main use case is for embedded where std is not available.
We can fix this by just potentially leaking memory (not calling drop) instead if it panics during drop.

[sgued]

Are you sure that IndexMap has the bug? It depends on Vec which does not appear to have it.

[sgued]

Oh, right it could lead to inconsistent state indeed even though it would not cause unsoundness in the case of IndexMap.

I'm fixing all this in #647