Implementation of iterator method fails for empty collections

[phronmophobic]

An easy way to reproduce is as follows:

tiara.data> (into [] (ordered-set))
Execution error (NullPointerException) at tiara.data.VecSet/iterator (data.clj:201).
Cannot invoke "java.util.Collection.iterator()" because the return value of "clojure.lang.IFn.invoke(Object)" is null
tiara.data> (into [] (ordered-map))
Execution error (NullPointerException) at tiara.data.VecMap/iterator (data.clj:58).
Cannot invoke "java.util.Collection.iterator()" because the return value of "clojure.lang.IFn.invoke(Object)" is null

I would be happy to submit a pull request, but I'm not actually sure what kind of fix would make sense.

Thanks for the library!

[phronmophobic]

Here is the workaround I'm currently using, phronmophobic@897d247. Not sure if it's a good solution, but it passes all the tests.