Make transform iterator save underlying iterator category.

[gogagum]

• operator[] in iterator facade now returns a reference, because its return type must be the same as dereference type for random_access_iterator concept.
• add tests for categories checks.
• change trait to determine transform_iterator reference type.

[Lastique]

I'm not sure what problem you're solving, but the reference type may not be an actual reference if the transform function returns an rvalue.

[Lastique]

I don't think I will accept these changes as they change the behavior of iterator_facade in an incompatible way and may break user-defined iterators. As it evidently broke counting_iterator. And I also don't think users should be required to define the operator[] (or other operators) themselves - it's the whole reason iterator_facade exists.

If you want operator[] to return an actual reference, a different approach is needed. iterator_facade needs to know whether returning a reference is safe and only then do that. I'm not sure how to do that other than with some new template parameter or a trait (that defaults to false for backward compatibility).

And finally, I'm not sure why you're changing the iterator category of the transform_iterator, but changing output_iterator_tag to input_iterator_tag seems entirely wrong to me.