Add ordered set operations?

Since, as noted in the `ordered-set` docstring, `clojure.set/union` and other operations in `clojure.set` may reorder elements for efficiency, perhaps it would be useful for `ordered` to include (less efficient?) set operations that preserve order. For example, I needed to preserve order with union, so I am using this definition (which might also be called "union"):
```clojure
(defn multi-conj
"Successively conj each element of ys onto xs."
[xs ys]
(reduce (fn [newxs y] (conj newxs y))
xs ys))
```
(I need it for two-element sets on the right, so efficiency shouldn't be an issue in my case.)