Aliasing violations in splice, mutable iteration and cursors

Summary

Four code paths create a pointer whose permission is later revoked, then write through it. Miri reports each as undefined behaviour under both the Tree Borrows and Stacked Borrows aliasing models (CWE-758). All four are reachable from safe code with standard `Ord` types.

1. **Splice back-pointer.** `split_off`, `split_off_index` and `append` stored the first spliced node's `prev` pointer from a reference to the head, then wrote through that same reference, invalidating the stored pointer. A later `pop_first` or `pop_front` writes through it. This affects all four collections. 2. **`SkipMap::range_mut` back pointer.** With an `Included` or `Excluded` upper bound, the back pointer came from a shared reference. `next_back` hands out `&mut V` through it, and writing to that value is undefined behaviour. 3. **`IterMut::next_back` reads a yielded node.** In `SkipList` and `SkipMap`, `next_back` read the predecessor node even after the front end had yielded it as `&mut`. The read freezes the caller's reference, and the caller's next write through it is undefined behaviour. This affects `iter_mut`, `values_mut` and `range_mut` used from both ends. 4. **Cursor list pointer.** `lower_bound_mut` and `upper_bound_mut` on `OrderedSkipList` and `SkipSet` (`cursor` feature) took the cursor's list pointer, then searched through `&self`. A comparator with interior-mutable state, such as a `Cell` call counter, wrote to the list's memory through that shared borrow and invalidated the cursor's pointer. The comparator can be a fully consistent order. `SkipMap` cursors are not affected.

No miscompilation has been observed in an optimised build. A future compiler may still optimise on the violated aliasing assumptions.

The flaws were corrected in 1.3.0. Each fix derives the pointer used for mutation from the collection's long-lived allocation pointers, and each has a Miri regression test.