Everywhere: Move global Kernel pattern code to Kernel/Library directory

This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.

Also, move the panic and assertions handling code to that directory.

Kernel/Library/Panic.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Format.h>
+
+namespace Kernel {
+
+[[noreturn]] void __panic(char const* file, unsigned int line, char const* function);
+
+#define PANIC(...)                                        \
+    do {                                                  \
+        critical_dmesgln("KERNEL PANIC! :^(");            \
+        critical_dmesgln(__VA_ARGS__);                    \
+        __panic(__FILE__, __LINE__, __PRETTY_FUNCTION__); \
+    } while (0)
+
+}