Rollup merge of #74762 - ssomers:btree_no_root_in_remove_kv_tracking, r=Mark-Simulacrum

BTreeMap::drain_filter should not touch the root during iteration

Although Miri doesn't point it out, I believe there is undefined behaviour using `drain_filter` when draining the 11th-last element from a tree that was larger. When this happens, the last remaining child nodes are merged, the root becomes empty and is popped from the tree. That last step establishes a mutable reference to the node elected root and writes a pointer in `node::Root`, while iteration continues to visit the same node.

This is mostly code from #74437, slightly adapted.

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