It make enum return the array of cases

IntRawValueEnumerable.swift

/**
adapt to enum which is Int Raw Value.
It make enum return the array of cases

#### Example
```
enum SomeEnum: IntRawValueEnumerable {
    case AAA
    case BBB
}

SomeEnum.allValues // => [.AAA, .BBB]
```
*/
protocol IntRawValueEnumerable {
    init?(rawValue: Int)
    var rawValue: Int { get }
}

extension IntRawValueEnumerable {
    static var generator: AnyGenerator<Self> {
        var index = start
        return anyGenerator {
            defer { index += step }    
            let next = Self(rawValue: index)
            return next
        }
    }

    static var allValues: [Self] {
        return Array(generator)
    }

    var start: Int { return 0 }
    var step:  Int { return 1 }
}