Reorganize stdlib-js sources specific to the current JS backend

Move kotlin-stdlib-js project and the sources specific to the current backend to 'stdlib/js-v1' directory,
but leave sources that can be shared with the new IR backend in the common 'stdlib/js' location
with exception for 'stdlib/js/src/generated', which is used exclusively for current backend.
This simplifies sourceset configuration when building stdlib with the new backend.

libraries/stdlib/js-ir/smallRuntime/smallRuntimeMissingDeclarations.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2019 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
+
+/**
+ * Returns `true` if this char sequence is empty (contains no characters).
+ */
+@kotlin.internal.InlineOnly
+public inline fun CharSequence.isEmpty(): Boolean = length == 0
+
+/**
+ * Accumulates value starting with [initial] value and applying [operation] from left to right to current accumulator value and each element.
+ */
+public inline fun <T, R> Array<out T>.fold(initial: R, operation: (acc: R, T) -> R): R {
+    var accumulator = initial
+    for (element in this) accumulator = operation(accumulator, element)
+    return accumulator
+}