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::next_back
methods temporarily create an exclusive reference to the key when
dereferencing the internal node pointer.
This invalidates the shared pointer held by the internal HashMap,
violating Stacked Borrows rules.
See advisory page for additional details.