Generate built-in sources of arrays

Extract Array class into a separate file, move arrayOfNulls to Library.kt, fix
formatting and generate Arrays.kt

core/builtins/native/kotlin/Array.kt

@@ -0,0 +1,10 @@
+package kotlin
+
+public class Array<reified T>(public val size: Int, init: Function1<Int, T>) {
+    public fun get(index: Int): T
+    public fun set(index: Int, value: T): Unit
+
+    public fun iterator(): Iterator<T>
+
+    public val indices: IntRange
+}