ObjC: provide macro annotation for memory management of NSObject used in case classes

```scala
@retain
case class Foo(id: NSInteger, date: NSDate, notes: String)
```
should transpile to
```scala
case class Foo(id: NSInteger, date: NSDate, notes: String) extends Releasable {
  date.retain()
  def release(): Unit = {
    date.release()
  }
```