Add ::: to NonEmptyList (#1949)

* Add ::: to NonEmptyList

* Add ++: to NEV and some doc to ::: in NEL

* Add doctest for NEL.::: and NEV.++:

* Use concatNel instead of concat

* Fix concat reference in scaladoc

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

@@ -99,6 +99,19 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {
   def prepend[AA >: A](a: AA): NonEmptyList[AA] =
     NonEmptyList(a, head :: tail)
 
+  /**
+    * Alias for concat
+    *
+    * {{{
+    * scala> import cats.data.NonEmptyList
+    * scala> val nel = NonEmptyList.of(1, 2, 3)
+    * scala> nel ::: NonEmptyList.of(4, 5)
+    * res0: cats.data.NonEmptyList[Int] = NonEmptyList(1, 2, 3, 4, 5)
+    * }}}
+    */
+  def :::[AA >: A](other: NonEmptyList[AA]): NonEmptyList[AA] =
+    other.concatNel(this)
+
   /**
    * Remove elements not matching the predicate
    *

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

@@ -71,6 +71,18 @@ final class NonEmptyVector[+A] private (val toVector: Vector[A]) extends AnyVal
    */
   def ++[AA >: A](other: Vector[AA]): NonEmptyVector[AA] = concat(other)
 
+  /**
+    * Append this NEV to another NEV, producing a new `NonEmptyVector`.
+    *
+    * {{{
+    * scala> import cats.data.NonEmptyVector
+    * scala> val nev = NonEmptyVector.of(1, 2, 3)
+    * scala> nev ++: NonEmptyVector.of(4, 5)
+    * res0: cats.data.NonEmptyVector[Int] = NonEmptyVector(1, 2, 3, 4, 5)
+    * }}}
+    */
+  def ++:[AA >: A](other: NonEmptyVector[AA]): NonEmptyVector[AA] = other.concatNev(this)
+
   /**
    * Append another `Vector` to this, producing a new `NonEmptyVector`.
    */