Radnom string generation

randomString.swift

// based on https://gist.github.com/szhernovoy/276e69eb90a0de84dd90
// and my performance measurements
public func randomString(of length: Int) -> String {
    let letters = Array("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
    var s = ""
    for _ in 0 ..< length {
        s.append(letters.randomElement()!)
    }
    return s
}