CaseIterable for older swift versions

CaseIterable.swift

#if !swift(>=4.2)
  public protocol CaseIterable {
    associatedtype AllCases: Collection where AllCases.Element == Self
    static var allCases: AllCases { get }
  }
  
  extension CaseIterable where Self: Hashable {
    static var allCases: [Self] {
      return [Self](AnySequence { () -> AnyIterator<Self> in
        var raw = 0
        var first: Self?
        return AnyIterator {
          let current = withUnsafeBytes(of: &raw) { $0.load(as: Self.self) }
          if raw == 0 {
            first = current
          } else if current == first {
            return nil
          }
          raw += 1
          return current
        }
      })
    }
  }
#endif