Add size to Foldable

Override with a more performant implementation in Map, Set, and Vector using the
size method provided by the standard library.

see typelevel#1091

core/src/main/scala/cats/Foldable.scala

@@ -1,6 +1,7 @@
 package cats
 
 import scala.collection.mutable
+import cats.std.long._
 import simulacrum.typeclass
 
 /**
@@ -56,6 +57,16 @@ import simulacrum.typeclass
       }
     }
 
+  /**
+   * The size of this Foldable.
+   *
+   * This is overriden in structures that have more efficient size implementations
+   * (e.g. Vector, Set, Map).
+   *
+   * Note: will not terminate for infinite-sized collections.
+   */
+  def size[A](fa: F[A]): Long = foldMap(fa)(_ => 1)
+
   /**
    * Fold implemented using the given Monoid[A] instance.
    */

core/src/main/scala/cats/std/map.scala

@@ -45,6 +45,8 @@ trait MapInstances extends cats.kernel.std.MapInstances {
       def foldRight[A, B](fa: Map[K, A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
         Foldable.iterateRight(fa.values.iterator, lb)(f)
 
+      override def size[A](fa: Map[K, A]): Long = fa.size.toLong
+
       override def isEmpty[A](fa: Map[K, A]): Boolean = fa.isEmpty
     }
 }

core/src/main/scala/cats/std/set.scala

@@ -18,6 +18,8 @@ trait SetInstances extends cats.kernel.std.SetInstances {
       def foldRight[A, B](fa: Set[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
         Foldable.iterateRight(fa.iterator, lb)(f)
 
+      override def size[A](fa: Set[A]): Long = fa.size.toLong
+
       override def exists[A](fa: Set[A])(p: A => Boolean): Boolean =
         fa.exists(p)
 

core/src/main/scala/cats/std/vector.scala

@@ -44,6 +44,8 @@ trait VectorInstances extends cats.kernel.std.VectorInstances {
         Eval.defer(loop(0))
       }
 
+      override def size[A](fa: Vector[A]): Long = fa.size.toLong
+
       def traverse[G[_], A, B](fa: Vector[A])(f: A => G[B])(implicit G: Applicative[G]): G[Vector[B]] =
       foldRight[A, G[Vector[B]]](fa, Always(G.pure(Vector.empty))){ (a, lgvb) =>
         G.map2Eval(f(a), lgvb)(_ +: _)

tests/src/test/scala/cats/tests/FoldableTests.scala

@@ -10,6 +10,12 @@ abstract class FoldableCheck[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
 
   def iterator[T](fa: F[T]): Iterator[T]
 
+  test("size") {
+    forAll { (fa: F[Int]) =>
+      fa.size should === (iterator(fa).size.toLong)
+    }
+  }
+
   test("summation") {
     forAll { (fa: F[Int]) =>
       val total = iterator(fa).sum