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/box/controlStructures/forIntRange.kt

@@ -1,13 +0,0 @@
-fun box() : String {
-    val a = arrayOfNulls<String>(3)
-    a[0] = "a"
-    a[1] = "b"
-    a[2] = "c"
-
-    var result = 0
-    for(i in a.indices) {
-      result += i
-    }
-    if (result != 3) return "FAIL"
-    return "OK"
-}

compiler/testData/codegen/box/controlStructures/kt998.kt

@@ -1,34 +0,0 @@
-fun findPairless(a : IntArray) : Int {
-  @loop for (i in a.indices) {
-    for (j in a.indices) {
-      if (i != j && a[i] == a[j]) continue@loop
-    }
-    return a[i]
-  }
-  return -1
-}
-
-fun hasDuplicates(a : IntArray) : Boolean {
-  var duplicate = false
-  @loop for (i in a.indices) {
-    for (j in a.indices) {
-      if (i != j && a[i] == a[j]) {
-        duplicate = true
-        break@loop
-      }
-    }
-  }
-  return duplicate
-}
-
-fun box() : String {
-    val a = IntArray(5)
-    a[0] = 0
-    a[1] = 0
-    a[2] = 1
-    a[3] = 1
-    a[4] = 5
-    if(findPairless(a) != 5) return "fail"
-    return if(hasDuplicates(a))  "OK" else "fail"
-
-}