add void for Optional, traversal, Fold and Setter

core/src/main/scala/monocle/Fold.scala

@@ -136,6 +136,10 @@ object Fold extends FoldInstances {
         s.fold(f,f)
     }
 
+  /** [[Fold]] that points to nothing */
+  def void[S, A]: Fold[S, A] =
+    Optional.void.asFold
+
   /** create a [[Fold]] from a [[Foldable]] */
   def fromFoldable[F[_]: Foldable, A]: Fold[F[A], A] =
     new Fold[F[A], A] {

core/src/main/scala/monocle/Optional.scala

@@ -236,6 +236,10 @@ object Optional {
   def codiagonal[S]: Optional[S \/ S, S] =
     POptional.codiagonal
 
+  /** [[Optional]] that points to nothing */
+  def void[S, A]: Optional[S, A] =
+    Optional[S, A](_ => None)(_ => identity)
+
   /** alias for [[POptional]] apply restricted to monomorphic update */
   def apply[S, A](_getOption: S => Option[A])(_set: A => S => S): Optional[S, A] =
     new Optional[S, A]{

core/src/main/scala/monocle/Setter.scala

@@ -127,6 +127,10 @@ object Setter {
   def codiagonal[S]: Setter[S \/ S, S] =
     PSetter.codiagonal
 
+  /** [[Setter]] that points to nothing */
+  def void[S, A]: Setter[S, A] =
+    Optional.void.asSetter
+
   /** alias for [[PSetter]] apply with a monomorphic modify function */
   def apply[S, A](modify: (A => A) => S => S): Setter[S, A] =
     PSetter(modify)

core/src/main/scala/monocle/Traversal.scala

@@ -222,6 +222,10 @@ object Traversal {
   def codiagonal[S, T]: Traversal[S \/ S, S] =
     PTraversal.codiagonal
 
+  /** [[Traversal]] that points to nothing */
+  def void[S, A]: Traversal[S, A] =
+    Optional.void.asTraversal
+
   def apply2[S, A](get1: S => A, get2: S => A)(set: (A, A, S) => S): Traversal[S, A] =
     PTraversal.apply2(get1, get2)(set)