Optimize text view and text iterator size by declaring the encoding state as a data member rather than deriving from it when doing so is beneficial

[tahonermann]

Modify text_view and text iterators to derive from the encoding state type only if the state type is an empty class and to declare a non-static data member otherwise. This will reduce the size of text_view and text iterator objects when the encoding state type is non-empty and has lower alignment requirements than the first non-static data member of the text_view and text iterator types.