Backports typelevel#3133 to scala_2.11

* Adds scala version specific src directories
* Backports PR typelevel#3133 to add scala 2.11 support
* Brings scalafmt plugin and configs utd with master

.scalafmt.conf

@@ -6,4 +6,4 @@ continuationIndent.defnSite = 2
 assumeStandardLibraryStripMargin = true
 danglingParentheses = true
 rewrite.rules = [AvoidInfix, SortImports, RedundantBraces, RedundantParens, SortModifiers]
-docstrings = JavaDoc
+docstrings = JavaDoc

core/src/main/scala-2.11/cats/data/ScalaVersionSpecificNonEmptyChainImpl.scala

@@ -0,0 +1,6 @@
+package cats.data
+
+private[data] trait ScalaVersionSpecificNonEmptyChainImpl {
+  def fromSeq[A](as: scala.collection.Seq[A]): Option[NonEmptyChain[A]] =
+    if (as.nonEmpty) Option(NonEmptyChainImpl.create(Chain.fromSeq(as))) else None
+}

core/src/main/scala-2.12/cats/data/ScalaVersionSpecificNonEmptyChainImpl.scala

@@ -0,0 +1,6 @@
+package cats.data
+
+private[data] trait ScalaVersionSpecificNonEmptyChainImpl {
+  def fromSeq[A](as: scala.collection.Seq[A]): Option[NonEmptyChain[A]] =
+    if (as.nonEmpty) Option(NonEmptyChainImpl.create(Chain.fromSeq(as))) else None
+}

core/src/main/scala-2.13+/cats/data/ScalaVersionSpecificNonEmptyChainImpl.scala

@@ -0,0 +1,3 @@
+package cats.data
+
+private[data] trait ScalaVersionSpecificNonEmptyChainImpl

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

@@ -21,9 +21,7 @@ import NonEmptyChainImpl.create
 import cats.{Order, Semigroup}
 import cats.kernel._
 
-import scala.collection.immutable._
-
-private[data] object NonEmptyChainImpl extends NonEmptyChainInstances {
+private[data] object NonEmptyChainImpl extends NonEmptyChainInstances with ScalaVersionSpecificNonEmptyChainImpl {
   // The following 3 types are components of a technique to
   // create a no-boxing newtype. It's coped from the
   // newtypes lib by @alexknvl
@@ -52,7 +50,7 @@ private[data] object NonEmptyChainImpl extends NonEmptyChainInstances {
   def fromNonEmptyVector[A](as: NonEmptyVector[A]): NonEmptyChain[A] =
     create(Chain.fromSeq(as.toVector))
 
-  def fromSeq[A](as: Seq[A]): Option[NonEmptyChain[A]] =
+  def fromSeq[A](as: scala.collection.immutable.Seq[A]): Option[NonEmptyChain[A]] =
     if (as.nonEmpty) Option(create(Chain.fromSeq(as))) else None
 
   def fromChainPrepend[A](a: A, ca: Chain[A]): NonEmptyChain[A] =

tests/src/test/scala/cats/tests/NonEmptyChainSuite.scala

@@ -106,7 +106,7 @@ class NonEmptyChainSuite extends CatsSuite {
 
   test("fromSeq . toList . iterator is id") {
     forAll { (ci: NonEmptyChain[Int]) =>
-      NonEmptyChain.fromSeq(ci.iterator.toList) should ===(Option(ci))
+      NonEmptyChain.fromSeq(ci.iterator.toSeq) should ===(Option(ci))
     }
   }