Add consistency laws for .combineAll and .combineAllOption.

These tests will catch sitautions where the optimized methods are
broken (i.e. become inconsistent with the behavior of .combine).
These tests caught an issue with Monoid[Map[K, V]].combineAll, which
was broken.

Fixes typelevel#1346.

kernel-laws/src/main/scala/cats/kernel/laws/GroupLaws.scala

@@ -1,6 +1,8 @@
-package cats.kernel.laws
+package cats.kernel
+package laws
 
 import cats.kernel._
+import cats.kernel.instances.option._
 
 import org.typelevel.discipline.Laws
 
@@ -27,7 +29,10 @@ trait GroupLaws[A] extends Laws {
     Rules.serializable(A),
     Rules.associativity(A.combine),
     Rules.repeat1("combineN")(A.combineN),
-    Rules.repeat2("combineN", "|+|")(A.combineN)(A.combine)
+    Rules.repeat2("combineN", "|+|")(A.combineN)(A.combine),
+    "combineAllOption" -> forAll { (xs: Vector[A]) =>
+      A.combineAllOption(xs) ?== xs.reduceOption(A.combine)
+    }
   )
 
   def band(implicit A: Band[A]): GroupProperties = new GroupProperties(
@@ -55,7 +60,10 @@ trait GroupLaws[A] extends Laws {
     Rules.rightIdentity(A.empty)(A.combine),
     Rules.repeat0("combineN", "id", A.empty)(A.combineN),
     Rules.collect0("combineAll", "id", A.empty)(A.combineAll),
-    Rules.isId("isEmpty", A.empty)(A.isEmpty)
+    Rules.isId("isEmpty", A.empty)(A.isEmpty),
+    "combineAll" -> forAll { (xs: Vector[A]) =>
+      A.combineAll(xs) ?== (A.empty +: xs).reduce(A.combine)
+    }
   )
 
   def commutativeMonoid(implicit A: CommutativeMonoid[A]): GroupProperties = new GroupProperties(

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

@@ -44,7 +44,7 @@ class MapMonoid[K, V](implicit V: Semigroup[V]) extends Monoid[Map[K, V]]  {
       val it = m.iterator
       while (it.hasNext) {
         val (k, v) = it.next
-        m.updated(k, Semigroup.maybeCombine(m.get(k), v))
+        acc(k) = Semigroup.maybeCombine(acc.get(k), v)
       }
     }
     StaticMethods.wrapMutableMap(acc)