This adds a counter and a 'dead' flag to the data stored in the linked
list. The counter counts the number of iterators (still) pointing to an
element and the dead flag is set to indicate the element was erased when
iterators are still pointing to it. As a result, iterators are NEVER
invalidated. Of course, such elements are subsequentially skipped when
iterating over the list. Assertions protect against dereferencing an
erased iterator, but incrementing or decremention still works: it is
still well-defined what the next (non erased) element is, assuming the
element wasn't erased (yet), but would be erased delayed - or assuming
the iterator would have been incremented (decremented) in advance to
erasing the element.
