Functor strengthL and strengthR

Add tests

core/src/main/scala/cats/Functor.scala

@@ -52,6 +52,16 @@ import simulacrum.typeclass
    */
   def as[A, B](fa: F[A], b: B): F[B] = map(fa)(_ => b)
 
+  /**
+    * Tuples the `A` value in `F[A]` with the supplied `B` value, with the `B` value on the left.
+    */
+  def strengthL[A, B](fa: F[A], b: B): F[(B, A)] = map(fa)(a => (b, a))
+
+  /**
+    * Tuples the `A` value in `F[A]` with the supplied `B` value, with the `B` value on the right.
+    */
+  def strengthR[A, B](fa: F[A], b: B): F[(A, B)] = map(fa)(a => (a, b))
+
   def compose[G[_]: Functor]: Functor[λ[α => F[G[α]]]] =
     new ComposedFunctor[F, G] {
       val F = self

tests/src/test/scala/cats/tests/FunctorTests.scala

@@ -18,6 +18,17 @@ class FunctorTest extends CatsSuite {
     }
   }
 
+  test("strengthL/R tuple values with a constant value preserving structure") {
+    forAll { (l: List[Int], o: Option[Int], m: Map[String, Int], i: Int) =>
+      l.strengthL(i) should === (List.tabulate(l.length)(in => (i, l(in))))
+      o.strengthL(i) should === (if (o.nonEmpty) Some((i, o.get)) else None)
+      m.strengthL(i) should === (m.map { case (k, v) => (k, (i, v)) }.toMap)
+      l.strengthR(i) should === (List.tabulate(l.length)(in => (l(in), i)))
+      o.strengthR(i) should === (if (o.nonEmpty) Some((o.get, i)) else None)
+      m.strengthR(i) should === (m.map { case (k, v) => (k, (v, i)) }.toMap)
+    }
+  }
+
   test("widen equals map(identity)") {
     forAll { (i: Int) =>
       val list: List[Some[Int]] = List(Some(i))