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

compiler/testData/codegen/boxWithStdlib/arrays/indices.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    val a = Array<Int>(5, {it})
+    val x = a.indices.iterator()
+    while (x.hasNext()) {
+        val i = x.next()
+        if (a[i] != i) return "Fail $i ${a[i]}"
+    }
+    return "OK"
+}

compiler/testData/codegen/boxWithStdlib/arrays/indicesChar.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    val a = CharArray(5)
+    val x = a.indices.iterator()
+    while (x.hasNext()) {
+        val i = x.next()
+        if (a[i] != 0.toChar()) return "Fail $i ${a[i]}"
+    }
+    return "OK"
+}