Add stack collection type

[AquaGeek]

This is mostly cribbed from the Swift book, but I figured it'd be nice to add a couple of these standard collection types to Foundation instead of everyone reimplementing them.

[harlanhaskins]

I feel like this could just be an extension on Array

extension Array {
    mutating func push(element: Element) {
        self.append(element)
    }

    func peek() -> Element? {
        return self.last
    }

    mutating func pop() -> Element? {
        if !self.isEmpty {
            return storage.removeLast()
        } else {
            return nil
        }
    }
}

You get all the same behavior, plus it'll still conform to CollectionType and the like.

[parkera]

This pull request contains an API change, so let’s follow the Swift evolution process, starting with discussing the idea on the swift-evolution and swift-corelibs-dev lists.