Support Swift 5 and Xcode 10.2

• Swift 4.2 / 5.0
• Replaced @_specialize with @inlinable
• pngData() and jpegData(compressionQuality:) similar to UIImage's
  • func data(using format: Image.Format) -> Data? was renamed
• Removed CountableRange and CountableClosedRange
• Renamed _Numeric to _NumericPixel and refactor _NumericPixel
• Removed APIs for images with channels of UInt32 and UInt64
• Removed RGBA<UInt8/16>.init for channels of Int

Changes from 0.6.0-alpha

• Fixed a bug of ImageProtocol.map in Swift 5