Support wrapper inheritance

[alexeyr]

Currently we have

 def arr: Rep[WArray[scala.Tuple2[A, B]]] = WPredef.genericArrayOps[A](PairCol.this.as.arr).zip[A, B, WArray[scala.Tuple2[A, B]]](/*WPredef.genericWrapArray[B](PairCol.this.bs.arr)*/???)(WArray.canBuildFrom[scala.Tuple2[A, B]](((element[scala.Tuple2[A, B]]): Elem[scala.Tuple2[A, B]])).asRep[WCanBuildFrom[WArray[A], scala.Tuple2[A, B], WArray[scala.Tuple2[A, B]]]]);

in https://github.com/scalan/scalanizer-demo/blob/0a8ca8e436b377805b96ef70ecb1c2035d9121e8/src/main/scala/scalanizer/collections/impl/ColsImpl.scala#L478 because the commented out version doesn't compile: WWrappedArray doesn't extend WGenIterable. If we have two wrappers of a subtype and a supertype (possibly indirectly) the subtype wrapper should extend the supertype wrapper.