Update extension syntax in scodec

[nicolasstucki]

This should unblock #9476.

The trick was to invert the arguments of the :: extension and give one of them a DummyImplicit.
Not sure if the inversion of arguments was intentional, we should double-check this behavior.

[odersky]

Yes, in fact the inversion of arguments is necessary. The old syntax used flipped parameters for
single extension methods relative to collective ones. So it was:

extension on (xs: List[Int]):
  def :: (x: Int): List[Int]

but

def (x: Int) :: (xs::List[Int]): List[Int]

The new extension syntax uses the natural x then xs order throughout. I.e.

extension (x: Int)
  def (xs: List[Int]): List[Int]

Great that you spotted this!