Remove mutable builder from Map traverse as its unlawful

Author: TimWSpence

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

@@ -46,14 +46,9 @@ trait MapInstances extends cats.kernel.instances.MapInstances {
         else
           G match {
             case x: StackSafeMonad[G] =>
-              x.map(fa.foldLeft(G.pure(Map.newBuilder[K, B])) { case (accG, (k, a)) =>
-                x.flatMap(accG) { acc =>
-                  G.map(f(a)) { a =>
-                    acc += k -> a
-                    acc
-                  }
-                }
-              })(_.result())
+              fa.iterator.foldLeft(G.pure(Map.empty[K, B])) { case (accG, (k, a)) =>
+                x.map2(accG, f(a)) { case (acc, b) => acc + (k -> b) }
+              }
             case _ =>
               G.map(Chain.traverseViaChain(fa.toIndexedSeq) { case (k, a) =>
                 G.map(f(a))((k, _))