Use-after-free in `clear` and `retain` when an element's `Drop` panics

SkipList::clear drops the node chain and only then resets tail and len. The drop runs each element's Drop, and T carries no bounds excluding a panicking one. If it unwinds, tail still points at the freed node while len stays non-zero. back, backmut, lastkeyvalue and last dereference tail through...