大写和小写字符串

要使字符串中的所有字符都为大写或小写：

Version = 2.2

let text = "AaBbCc"
let uppercase = text.uppercaseString // "AABBCC"
let lowercase = text.lowercaseString // "aabbcc"

Version = 3.0

let text = "AaBbCc"
let uppercase = text.uppercased() // "AABBCC"
let lowercase = text.lowercased() // "aabbcc"