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/runtime/misc.kt

@@ -0,0 +1,10 @@
+/*
+ * 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.js
+
+// TODO: Polyfill
+internal fun imul(a_local: Int, b_local: Int) =
+    js("((a_local & 0xffff0000) * (b_local & 0xffff) + (a_local & 0xffff) * (b_local | 0)) | 0").unsafeCast<Int>()