added instances of BitSet to allInstances

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

@@ -14,6 +14,7 @@ trait AllInstances
   with    MapInstances
   with    BigIntInstances
   with    BigDecimalInstances
+  with    BitSetInstances
   with    FutureInstances
   with    TryInstances
   with    TupleInstances

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

@@ -0,0 +1,8 @@
+package cats.instances
+
+import scala.collection.immutable.BitSet
+import cats.Show
+
+trait BitSetInstances extends cats.kernel.instances.BitSetInstances {
+  implicit def catsStdShowForBitSet: Show[BitSet] = Show.fromToString[BitSet]
+}

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

@@ -9,6 +9,7 @@ package object instances {
   object list       extends ListInstances
   object option     extends OptionInstances
   object set        extends SetInstances
+  object bitSet     extends BitSetInstances
   object stream     extends StreamInstances
   object vector     extends VectorInstances
   object map        extends MapInstances

tests/src/test/scala/cats/tests/BitSetTests.scala

@@ -0,0 +1,21 @@
+package cats.tests
+
+import org.scalacheck.Arbitrary
+import org.scalacheck.Arbitrary.arbitrary
+
+import scala.collection.immutable.BitSet
+
+class BitSetTests extends CatsSuite {
+  implicit val arbitraryBitSet: Arbitrary[BitSet] =
+    Arbitrary(arbitrary[List[Short]].map(ns => BitSet(ns.map(_ & 0xffff): _*)))
+
+  test("show BitSet"){
+    BitSet(1, 1, 2, 3).show should === ("BitSet(1, 2, 3)")
+    BitSet.empty.show should === ("BitSet()")
+
+    forAll { fs: BitSet =>
+      fs.show should === (fs.toString)
+    }
+  }
+
+}