Row: it can now be iterated

[eutampieri]

I've added .iter() to the Row struct.

[IvanUkhov]

Thank you for the PR. In general, it might be helpful to have this possibility. However, I think it will be confusing if the iterator does not return the columns in the original order. I would propose to ensure that happens.

[eutampieri]

Yes, I was trying to do what you said, and I originally thought that the Vec contains the columns in the right order, but it seems to be random.
Let me know if you have any ideas on how to preserve (or pass on) the columns order.
Also, thanks for reviewing this PR so quickly!

[IvanUkhov]

One could wrap column_names into Rc and then put it into Row next to column_mapping. One could then iterate over column_names instead of column_mapping.

[IvanUkhov]

Thank you!