SI-7187 deprecate eta-expansion of arity 0

Lukas says in https://github.com/scala/scala/pull/4822#issuecomment-157013973:

```
object Test {
  def foo(): () => String = () => ""
  val f: () => Any = foo

  def main(args: Array[String]): Unit = {
    println(f()) // <function0>
  }
}
```

Should we emit a deprecation in this case? 
