Disable transcoding in utf_iterator if no transcoding is necessary.

[andreasbuhr]

Another option to implement
#260

[tzlaine]

This is the kind of thing I'd like to avoid -- it's pretty complicated for what it does. For instance, the FromFormat == ToFormat and FromFormat != ToFormat cases have different invariants. The read position is before the current cp for the former, and after it for the latter. I think it would be clearer to just pick the right iterator in the first place. IOW, the equivalent to the FromFormat != ToFormat check should happen at the point where we choose whether to use utf_iterator or not.