The 'step' method is split into two parts, 'peek' and 'push'. peek
returns the next state but doesn't make it current.
The end of iteration was previously tracked by setting 'trie' to nil.
End of iteration is now tracked using the 'iteratorEnd' error, which is
slightly cleaner and requires less code.
Make it so each iterator has exactly one public constructor:
- NodeIterators can be created through a method.
- Iterators can be created through NewIterator on any NodeIterator.
This PR implements a differenceIterator, which allows iterating over trie nodes
that exist in one trie but not in another. This is a prerequisite for most GC
strategies, in order to find obsolete nodes.
