feature: Make OrderedDictionary.Values conform to Differentiable

[JaapWijnen]

This makes OrderedDictionary.Values conform to the Differentiable protocol.
You can now access the values of an OrderedDictionary in a differentiable way:

@differentiable(reverse)
func example1(dict: OrderedDictionary<String, Double>) -> [Double] {
    dict.values.elements
}

subscripting also works:

@differentiable(reverse)
func example2(dict: OrderedDictionary<String, Double>, index: Int) -> Double {
    dict.values[index]
}

[clarkdobson]

Looks good, thanks for adding this functionality!

[clarkdobson]

Looks good as is, should be ready to merge.