GHSA-RHFX-M35P-FF5J `IterMut` violates Stacked Borrows by invalidating internal pointer

Affected versions of this crate contain a soundness issue in the IterMut iterator implementation. The IterMut::next and IterMut::nextback methods temporarily create an exclusive reference to the key when dereferencing the internal node pointer. This invalidates the shared pointer held by the...