Make Array.indices extension property, move to stdlib

This is not something that needs to be intrinsified. Note that compiler
optimizations are still possible and the fact whether 'indices' is a member or
an extension is irrelevant to the optimizer

core/builtins/native/kotlin/Array.kt

@@ -24,7 +24,5 @@ public class Array<reified T>(size: Int, init: (Int) -> T) : Cloneable {
 
     public fun iterator(): Iterator<T>
 
-    public val indices: IntRange
-
     public override fun clone(): Array<T>
 }