AK: Add a simple randomness API

This can be used in code that builds on non-Serenity platforms.

AK/Random.h

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Platform.h>
+#include <AK/Types.h>
+
+#if defined(__serenity__)
+#    include <stdlib.h>
+#endif
+
+#if defined(__linux__)
+#    include <unistd.h>
+#endif
+
+namespace AK {
+
+inline void fill_with_random(void* buffer, size_t length)
+{
+#if defined(__serenity__)
+    arc4random_buf(buffer, length);
+#elif defined(__linux__)
+    int rc = getentropy(buffer, length);
+    (void)rc;
+#endif
+}
+
+template<typename T>
+inline T get_random()
+{
+    T t;
+    fill_with_random(&t, sizeof(T));
+    return t;
+}
+
+}