Add default constructor for MapRef

std/shared/src/main/scala/cats/effect/std/MapRef.scala

@@ -45,6 +45,20 @@ trait MapRef[F[_], K, V] extends Function1[K, Ref[F, V]] {
 
 object MapRef extends MapRefCompanionPlatform {
 
+  /**
+   * Default constructor for [[MapRef]]. If [[Sync]] is available, it will delegate to
+   * [[ofConcurrentHashMap], otherwise it will fallback to [[ofShardedImmutableMap]].
+   */
+  def apply[F[_]: Concurrent, K, V]: F[MapRef[F, K, Option[V]]] = {
+    Concurrent[F] match {
+      case s: Sync[F] =>
+        ofConcurrentHashMap()(s)
+      case _ =>
+        ofShardedImmutableMap[F, K, V](shardCount = Runtime.getRuntime.availableProcessors())
+    }
+
+  }
+
   /**
    * Creates a sharded map ref to reduce atomic contention on the Map, given an efficient and
    * equally distributed hash, the contention should allow for interaction like a general
@@ -522,5 +536,8 @@ object MapRef extends MapRefCompanionPlatform {
           val (set, out) = f(v)
           (set.some, out.some)
       }
+
+    def withDefaultValue(default: V)(implicit E: Eq[V], F: Functor[F]): MapRef[F, K, V] =
+      defaultedMapRef(mRef, default)
   }
 }