Creates a new instance with the specified raw value.
If there is no value of the type that corresponds with the specified raw
value, this initializer returns nil. For example:
enumPaperSize:String{caseA4,A5,Letter,Legal}print(PaperSize(rawValue:"Legal"))// Prints "Optional(PaperSize.Legal)"print(PaperSize(rawValue:"Tabloid"))// Prints "nil"