Refact stdlib generator, add support for different backends

[JS IR BE] Runtime fixes
 * Do not generate external declarations for IR BE
 * Move `arrayToString` helper function out of shared JS stdlib
 * Fix arrays type check for IR BE

libraries/stdlib/js/irRuntime/generated/_CollectionsJs.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license 
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin.collections
+
+//
+// NOTE: THIS FILE IS AUTO-GENERATED by the GenerateStandardLib.kt
+// See: https://github.com/JetBrains/kotlin/tree/master/libraries/stdlib
+//
+
+import kotlin.js.*
+
+/**
+ * Reverses elements in the list in-place.
+ */
+public actual fun <T> MutableList<T>.reverse(): Unit {
+    val midPoint = (size / 2) - 1
+    if (midPoint < 0) return
+    var reverseIndex = lastIndex
+    for (index in 0..midPoint) {
+        val tmp = this[index]
+        this[index] = this[reverseIndex]
+        this[reverseIndex] = tmp
+        reverseIndex--
+    }
+}
+