Expose headOption for Chain (#2610)

* Expose headOption for Chain

* Address Whitespace In Line

* Test headOption behavior

* Scalafmt Chain Tests

* Incorporate Better Property Test

* Scalafmt again

core/src/main/scala/cats/data/Chain.scala

@@ -46,6 +46,11 @@ sealed abstract class Chain[+A] {
     result
   }
 
+  /**
+   * Returns the head of this Chain if non empty, none otherwise. Amortized O(1).
+   */
+  def headOption: Option[A] = uncons.map(_._1)
+
   /**
    * Returns true if there are no elements in this collection.
    */

tests/src/test/scala/cats/tests/ChainSuite.scala

@@ -56,6 +56,12 @@ class ChainSuite extends CatsSuite {
     }
   }
 
+  test("headOption") {
+    forAll { (s: Seq[Int]) =>
+      Chain.fromSeq(s).headOption should ===(s.headOption)
+    }
+  }
+
   test("size is consistent with toList.size") {
     forAll { (ci: Chain[Int]) =>
       ci.size.toInt should ===(ci.toList.size)