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

Summary

`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()`, `back_mut()`, `last_key_value()` and `last()` dereference `tail` through `unsafe`, so reading the container after the unwind is a use-after-free (CWE-416). `Drop for SkipList` calls `Box::from_raw` on `self.head`, which `clear` already destroyed, so dropping the container is a double free (CWE-415).

`retain`, `retain_mut` and `dedup_by` reach the same state through `Node::filter_rebuild`, which frees nodes and runs a user predicate before the caller commits `tail` and `len`. There the head links are left partially rewired, so traversal can also reach freed nodes.

## Mitigation

Update to 1.1.1.