Before this would assume that the element found in operator++ was still
valid when dereferencing it in operator*.
Since any code can have been run since that increment this is not always
valid.
To further simplify the logic of the iterator we no longer store the
index in an optional.
While this implementation should be complete it is based on HashMap's
iterator, which currently follows bucket-order instead of the required
insertion order. This can be simply fixed by replacing the underlying
HashMap member in Map with an enhanced one that maintains a linked
list in insertion order.
