Add optional variant of onEnum function

[ajoklar]

In Swift it is common to switch over an optional enum, i.e.:

enum Message {
    case text
    case image
}

let message: Message? = .text

switch message {
case .none: // handle none case
case .text: // handle text case
case .image: // handle image case
}

The function onEnum only accepts non-nil types, which is less ergonomic, because we have to nil-check before doing the switch.
It would be great if a function like this would be generated:

public func onEnum<__Sealed : Skie.MyModule.Message.__Kotlin>(of sealed: __Sealed?) -> Skie.MyModule.Message.__Sealed? {
   if sealed == nil {
    return nil
  } else // followed by the code that is already generated
}

[TadeasKriz]

Hey @ajoklar, this is a good idea. It does double the number of the overloads, so I want to first try and look into possible ways to have just one that'd call the specific onEnum when not nil.

[FilipDolnik]

Implemented in 0.6.0.