Add NonEmptyList.zipWithIndex (#1517)

* Add NonEmptyList.zipWithIndex

* Use builder instead of recursion for performance reasons.

core/src/main/scala/cats/data/NonEmptyList.scala

@@ -165,6 +165,27 @@ final case class NonEmptyList[A](head: A, tail: List[A]) {
     go(head, tail, Nil)
   }
 
+  /**
+   * Zips each element of this `NonEmptyList` with its index.
+   *
+   * {{{
+   * scala> import cats.data.NonEmptyList
+   * scala> val nel = NonEmptyList.of("a", "b", "c")
+   * scala> nel.zipWithIndex
+   * res0: cats.data.NonEmptyList[(String, Int)] = NonEmptyList((a,0), (b,1), (c,2))
+   * }}}
+   */
+  def zipWithIndex: NonEmptyList[(A, Int)] = {
+    val bldr = List.newBuilder[(A, Int)]
+    var idx = 1
+    val it = tail.iterator
+    while (it.hasNext) {
+      bldr += ((it.next, idx))
+      idx += 1
+    }
+    NonEmptyList((head, 0), bldr.result)
+  }
+
 }
 
 object NonEmptyList extends NonEmptyListInstances {

tests/src/test/scala/cats/tests/NonEmptyListTests.scala

@@ -218,6 +218,12 @@ class NonEmptyListTests extends CatsSuite {
       nel.reverse.toList should === (nel.toList.reverse)
     }
   }
+
+  test("NonEmptyList#zipWithIndex is consistent with List#zipWithIndex") {
+    forAll { nel: NonEmptyList[Int] =>
+      nel.zipWithIndex.toList should === (nel.toList.zipWithIndex)
+    }
+  }
 }
 
 class ReducibleNonEmptyListCheck extends ReducibleCheck[NonEmptyList]("NonEmptyList") {