Move built-ins from package "jet" to "kotlin"

#KT-2896 Fixed

core/builtins/src/kotlin/Unit.kt

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2010-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kotlin
+
+public class Unit private() {
+    override fun toString(): String = "Unit.VALUE"
+
+    class object {
+        public val VALUE: Unit = Unit()
+    }
+}