Sorted dict 1

Pull request for SortedDict and BalancedTree23 taking into account Kevin Squire's feedback from the past 12 days.
Steve, thanks for the explanation.  I think that's what I was trying to say tersely, so I do actually get it.  

Part of the reason I wanted Stefan's opinion was because he conceived of the `Order` classes originally.  Until now, these have only been used in sorting algorithms, and we've gotten away without something like `eq` by using versions of the various sorting algorithms which did not require it.  (This kind of surprised me originally, but the algorithms are all quite reasonable--I actually quite like the quicksort variant we use, although I had never seen it written that way before.)

@StefanKarpinski, I'm wondering if the definitions of `eq` shouldn't actually go in base?  They'd still need to be here for compatibility with v0.3, of course.