Gave SortedDict a sorted __repr__

A sorted container should have a sorted representation. Now it does. `SortedDict` now prints in sorted order, just like `SortedList` and `SortedSet`. Having a `SortedDict` iterate in perfect order but print in `dict`'s random order was disconcerting, to say the least. 

Also generalized class name used in `__repr__`, in case someone cares to subclass.