Move everything into the scala.colleciton.compat package

Fixes #30

Using `scala.collection.compat.imm` (not `.immutable`) to avoid
name clashes with `scala.collection.immutable`.

Author: lrytz

src/main/scala-2.11_2.12/scala/collection/BuildFrom.scala renamed to src/main/scala-2.11_2.12/scala/collection/compat/BuildFrom.scala

@@ -1,6 +1,7 @@
-package scala.collection
+package scala.collection.compat
 
 import scala.collection.generic.CanBuildFrom
+import scala.collection.{Iterable, mutable}
 
 /** Builds a collection of type `C` from elements of type `A` when a source collection of type `From` is available.
   * Implicit instances of `BuildFrom` are available for all collection types.

src/main/scala-2.11_2.12/scala/collection/compat/CompatImpl.scala

@@ -1,8 +1,8 @@
-package scala.collection.compat
+package scala.collection
+package compat
 
 import scala.collection.generic.CanBuildFrom
 import scala.collection.mutable.Builder
-import scala.collection.{immutable, mutable}
 
 private[compat] object CompatImpl {
   def simpleCBF[A, C](f: => Builder[A, C]): CanBuildFrom[Any, A, C] = new CanBuildFrom[Any, A, C] {

src/main/scala-2.11_2.12/scala/collection/Factory.scala renamed to src/main/scala-2.11_2.12/scala/collection/compat/Factory.scala

@@ -1,6 +1,7 @@
-package scala.collection
+package scala.collection.compat
 
 import scala.collection.generic.CanBuildFrom
+import scala.collection.{TraversableOnce, mutable}
 
 /**
   * A factory that builds a collection of type `C` with elements of type `A`.
@@ -33,4 +34,4 @@ object Factory {
   implicit def fromCanBuildFromConversion[X, A, C](x: X)(implicit toCanBuildFrom: X => CanBuildFrom[Nothing, A, C]): Factory[A, C] =
     fromCanBuildFrom(toCanBuildFrom(x))
 
-}
+}

src/main/scala-2.11_2.12/scala/collection/immutable/ArraySeq.scala renamed to src/main/scala-2.11_2.12/scala/collection/compat/imm/ArraySeq.scala

@@ -1,15 +1,14 @@
-package scala
-package collection
-package immutable
+package scala.collection.compat.imm
 
-import scala.reflect.ClassTag
-import scala.runtime.BoxedUnit
+import java.util.Arrays
+
+import scala.annotation.unchecked.uncheckedVariance
+import scala.collection.AbstractSeq
 import scala.collection.generic._
-import scala.collection.mutable.{Builder, ArrayBuilder, ArrayBuffer, WrappedArrayBuilder}
+import scala.collection.immutable.IndexedSeq
+import scala.collection.mutable.{ArrayBuilder, Builder, WrappedArrayBuilder}
+import scala.reflect.ClassTag
 import scala.util.hashing.MurmurHash3
-import scala.annotation.unchecked.uncheckedVariance
-
-import java.util.Arrays
 
 /**
   * An immutable array.
@@ -42,9 +41,6 @@ abstract class ArraySeq[+T]
   /** The underlying array */
   def unsafeArray: Array[T @uncheckedVariance]
 
-  private def elementClass: Class[_] =
-    unsafeArray.getClass.getComponentType
-
   override def stringPrefix = "ArraySeq"
 
   /** Clones this object, including the underlying Array. */

src/main/scala-2.11_2.12/scala/collection/compat/package.scala

@@ -1,7 +1,6 @@
 package scala.collection
 
 import scala.collection.generic._
-import scala.collection.mutable.Builder
 import scala.reflect.ClassTag
 
 /** The collection compatibility API */

src/main/scala-2.13/scala/collection/compat/imm/package.scala

@@ -0,0 +1,6 @@
+package scala.collection.compat
+
+package object imm {
+  type ArraySeq[+T] = scala.collection.immutable.ArraySeq[T]
+  val ArraySeq      = scala.collection.immutable.ArraySeq
+}

src/main/scala-2.13/scala/collection/compat/package.scala

@@ -1,5 +1,11 @@
 package scala.collection
 
-// Empty dummy package to allow cross-building with Scala 2.12
 package object compat {
+  type Factory[-A, +C] = scala.collection.Factory[A, C]
+  val Factory          = scala.collection.Factory
+
+  type BuildFrom[-From, -A, +C] = scala.collection.BuildFrom[From, A, C]
+  val BuildFrom                 = scala.collection.BuildFrom
+
+  type IterableOnce[+X] = scala.collection.IterableOnce[X]
 }

src/test/scala/test/scala/collection/ArraySeqTest.scala

@@ -2,7 +2,7 @@ package test.scala.collection
 
 import org.junit.{Assert, Test}
 
-import scala.collection.immutable.ArraySeq
+import scala.collection.compat.imm.ArraySeq
 
 // The unmodified ArraySeqTest from collection-strawman
 class ArraySeqTest {

src/test/scala/test/scala/collection/BuildFromTest.scala

@@ -5,7 +5,7 @@ import org.junit.Test
 import scala.collection.compat._
 import scala.collection.immutable.{HashMap, TreeMap, TreeSet}
 import scala.collection.mutable.{ArrayBuffer, Builder, ListBuffer}
-import scala.collection.{BitSet, BuildFrom, SortedMap, SortedSet, immutable, mutable}
+import scala.collection.{BitSet, SortedMap, SortedSet, immutable, mutable}
 
 // Tests copied from the 2.13 scala-library
 class BuildFromTest {

src/test/scala/test/scala/collection/FactoryTest.scala

@@ -3,7 +3,7 @@ package test.scala.collection
 import org.junit.{Assert, Test}
 
 import scala.collection.compat._
-import scala.collection.{BitSet, Factory, immutable, mutable}
+import scala.collection.{BitSet, immutable, mutable}
 
 class FactoryTest {