Improve Array.copy performance

If source and dest are both non-primitive array types, then we can
always use System.arraycopy and avoid the slowcopy
branch. ArrayBuffer.toArray is added to the benchmark suite in order
to avoid regressions and demonstrate the performance impact.

Author: bbrehm

library/src/scala/Array.scala

@@ -106,7 +106,9 @@ object Array {
    */
   def copy(src: AnyRef, srcPos: Int, dest: AnyRef, destPos: Int, length: Int): Unit = {
     val srcClass = src.getClass
-    if (srcClass.isArray && dest.getClass.isAssignableFrom(srcClass))
+    val destClass = dest.getClass
+    if (srcClass.isArray && ((destClass eq srcClass) ||
+        (destClass.isArray && !srcClass.getComponentType.isPrimitive && !destClass.getComponentType.isPrimitive)))
       java.lang.System.arraycopy(src, srcPos, dest, destPos, length)
     else
       slowcopy(src, srcPos, dest, destPos, length)