Fix in coproduct at signature

```scala
scala> val c = Coproduct[Int :+: Double :+: Boolean :+: CNil](0.0)
c: shapeless.:+:[Int,shapeless.:+:[Double,shapeless.:+:[Boolean,shapeless.CNil]]] = 0.0

scala> c.at(0)
<console>:12: error: overloaded method value at with alternatives:
  [N <: shapeless.Nat](n: N)(implicit at: shapeless.ops.coproduct.At[shapeless.:+:[Int,shapeless.:+:[Double,shapeless.:+:[Boolean,shapeless.CNil]]],n.N])Option[at.A] <and>
  [N <: shapeless.Nat](implicit at: shapeless.ops.coproduct.At[shapeless.:+:[Int,shapeless.:+:[Double,shapeless.:+:[Boolean,shapeless.CNil]]],N])Option[at.A]
 cannot be applied to (Int)
              c.at(0)
                ^
```

Looks a bit like https://github.com/milessabin/shapeless/pull/246, https://github.com/milessabin/shapeless/pull/209. But both overloads can be kept, the non-type argument one just needs some adjustments.