annotation @ poly ?

Transforms
```scala
@poly
trait MyTypeclass[In] {
  type Out
  def foo(in: In): Out
}
```
into
```scala
trait MyTypeclass[In] {
  type Out
  def foo(in: In): Out
}
object MyTypeclass {
  object myTypeclassPoly extends Poly1 {
    implicit def cse(implicit mtc: MyTypeclass[In]): Case.Aux[In, mtc.Out] = at(in => mtc.foo(in))
  }
}
```