| Return a new two level iterator. A two-level
| iterator contains an index iterator whose
| values point to a sequence of blocks where each
| block is itself a sequence of key,value pairs.
| The returned two-level iterator yields the
| concatenation of all key/value pairs in the
| sequence of blocks. Takes ownership of
| “index_iter” and will delete it when no longer
| needed.
|
| Uses a supplied function to convert an
| index_iter value into an iterator over the
| contents of the corresponding block.