Iterators are not bidirectional

[JohelEGP]

Greetings, Tim.

I want to let you know that fee8dbd and 0f6c8a1 introduced a number of changes to the iterators that prevent them from satisfying the requirements of BidirectionalIterator, which they claim to be via their iterator_category member alias. Now they are InputIterators at best.

[timniederhausen]

Thanks for letting me know, it should be fixed now.

Some of the more serious issues seem to have been present in the original version as well (operator--(int) actually incremented the iterator).