Add scanLeftTail to NonEmptyCollection.

Considering doing a scan on a non-empty collection yields a collection
with at least _two_ elements, this method helps users discover that
fact.

core/src/main/scala-2.13+/cats/data/NonEmptyLazyList.scala

@@ -277,6 +277,9 @@ class NonEmptyLazyListOps[A](private val value: NonEmptyLazyList[A])
  final def scanLeft[B](b: B)(f: (B, A) => B): NonEmptyLazyList[B] =
  create(toLazyList.scanLeft(b)(f))
 
+ final def scanLeftTail[B](b: B)(f: (B, A) => B): NonEmptyLazyList[B] =
+ create(toLazyList.scanLeft(b)(f).tail)
+
  /**
  * Left-associative reduce using f.
  */

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

@@ -313,6 +313,9 @@ class NonEmptyChainOps[A](private val value: NonEmptyChain[A])
  create(result)
  }
 
+ final def scanLeftTail[B](b: B)(f: (B, A) => B): NonEmptyChain[B] =
+ create(scanLeft(b)(f).tail)
+
  /**
  * Right-associative fold using f.
  */

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

@@ -47,6 +47,7 @@ private[cats] trait NonEmptyCollection[+A, U[+_], NE[+_]] extends Any {
 
  def foldLeft[B](b: B)(f: (B, A) => B): B
  def scanLeft[B](b: B)(f: (B, A) => B): NE[B]
+ def scanLeftTail[B](b: B)(f: (B, A) => B): NE[B]
  def reduce[AA >: A](implicit S: Semigroup[AA]): AA
 
  def zipWith[B, C](b: NE[B])(f: (A, B) => C): NE[C]

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

@@ -305,6 +305,9 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) extends NonEmptyCollec
  def scanLeft[B](b: B)(f: (B, A) => B): NonEmptyList[B] =
  NonEmptyList(b, tail.scanLeft(f(b, head))(f))
 
+ def scanLeftTail[B](b: B)(f: (B, A) => B): NonEmptyList[B] =
+ NonEmptyList.fromListUnsafe(tail.scanLeft(f(b, head))(f))
+
  /**
  * Left-associative reduce using f.
  */

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

@@ -181,6 +181,9 @@ final class NonEmptySeq[+A] private (val toSeq: Seq[A]) extends AnyVal with NonE
  def scanLeft[B](b: B)(f: (B, A) => B): NonEmptySeq[B] =
  new NonEmptySeq(toSeq.scanLeft(b)(f))
 
+ def scanLeftTail[B](b: B)(f: (B, A) => B): NonEmptySeq[B] =
+ new NonEmptySeq(toSeq.scanLeft(b)(f).tail)
+
  /**
  * Right-associative fold using f.
  */

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

@@ -191,6 +191,9 @@ final class NonEmptyVector[+A] private (val toVector: Vector[A])
  def scanLeft[B](b: B)(f: (B, A) => B): NonEmptyVector[B] =
  NonEmptyVector.fromVectorUnsafe(toVector.scanLeft(b)(f))
 
+ def scanLeftTail[B](b: B)(f: (B, A) => B): NonEmptyVector[B] =
+ NonEmptyVector.fromVectorUnsafe(tail.scanLeft(f(b, head))(f))
+
  /**
  * Right-associative fold using f.
  */