Generalized Number Pairs solution to unsorted arrays

TwoSum assumed that the received array was sorted, as the codeeval challenge
states, but it is also nice to give a general solution with non-sorted arrays.
Since it was already applying a transformation from Seq to Set, the change is
as easy as transforming the Seq to a SortedSet instead, and iterating over it
as opposed to doing it over the given (maybe unsorted) array.

Does not change score in codeeval, previous version was already 100% on it.