Don't use forwarders in closure implementation

[allanrenucci]

class Foo {
  (x: Int) => x + 1
}

A closure that is not a SAM will desugar to something like this in Dotty:

class Foo {
  def apply(x: Int) = x + 1

  class $anon extends Function1[Int, Int] {
    def apply(x: Int) = Foo.this.apply(x)
  }
  new $anon
}

and this in scalac:

class Foo {
  class $anon extends Function1[Int, Int] {
    def apply(x: Int) =  x + 1
  }
  new $anon
}

The issue with the Dotty desugaring, is that $anon will capture a reference to the outer class Foo that is not needed

[allanrenucci]

Here is an example where the captured outer reference prevents a partial function from being serialisable:

class Foo {
  def foo: PartialFunction[Int, Int] = { case x => x + 1 }
}

object Test {
  def serializeDeserialize[T <: AnyRef](obj: T): T = {
    import java.io._
    val buffer = new ByteArrayOutputStream
    val out = new ObjectOutputStream(buffer)
    out.writeObject(obj)
    val in = new ObjectInputStream(new ByteArrayInputStream(buffer.toByteArray))
    in.readObject.asInstanceOf[T]
  }

  def main(args: Array[String]): Unit = {
    val adder = serializeDeserialize((new Foo).foo)
    assert(adder(1) == 2)
  }
}

Exception in thread "main" java.io.NotSerializableException: Foo
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
	at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
	at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
	at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
	at Test$.serializeDeserialize(Test.scala:10)
	at Test$.main(Test.scala:16)
	at Test.main(Test.scala)

[allanrenucci]

Fixing #4438 would make this fix trivial