A simple Swift version of Scala's mkString

MakeString.swift

extension Collection {
    func makeString(_ separator: String) -> String { makeString("", separator, "") }

    func makeString(_ prefix: String, _ separator: String, _ suffix: String) -> String {
        if isEmpty { 
            return prefix + suffix 
        } else {
            return addString("", prefix, separator, suffix)
        }
    }

    private func addString(_ string: String, _ prefix: String, _ separator: String, _ suffix: String) -> String {
        var string = string
        if prefix.count != 0 { string.append(contentsOf: prefix) }
        var it = makeIterator() 
        if let start = it.next() {
            string.append(contentsOf: "\(start)")
            while let rest = it.next() {
                string.append(contentsOf: separator)
                string.append(contentsOf: "\(rest)")
            }
        }
        if suffix.count != 0 { string.append(contentsOf: suffix) }
        return string
    }
}