Provide common Random API

#KT-17261

libraries/stdlib/js/src/kotlin/random/PlatformRandom.kt

@@ -0,0 +1,24 @@
+/*
+ * 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.random
+
+import kotlin.math.pow
+
+internal actual fun defaultPlatformRandom(): Random =
+    XorWowRandom((js("Math").random().unsafeCast<Double>() * (2.0.pow(32.0))).toInt())
+
+
+internal actual fun fastLog2(value: Int): Int {
+    // TODO: not so fast, make faster
+    require(value > 0)
+    var v = value
+    var log = -1
+    while (v != 0) {
+        v = v.ushr(1)
+        log++
+    }
+    return log
+}