simplify iterators to avoid type merging juggling when order is not needed

Users who use getAllIndexedFSs(type) to iterate over all FeatureStructures of a particular type, in no particular order, for types which have subtypes, use the PointerIterator implementation which juggles the collection of iterators for each subtype and "merges" them in the proper order. This is done even when the index type is a BAG. Add an alternative implementation which does the much simpler thing for these cases (where order isn't needed) that avoids the juggling operations and just goes through each index sequentially.