BTreeMap: prevent tree from ever being owned by non-root node
This introduces a new marker type, `Dying`, which is used to note trees which are in the process of deallocation. On such trees, some fields may be in an inconsistent state as we are deallocating the tree. Unfortunately, there's not a great way to express conditional unsafety, so the methods for traversal can cause UB if not invoked correctly, but not marked as such. This is not a regression from the previous state, but rather isolates the destructive methods to solely being called on the dying state.
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