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/ScopedCritical.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Library/ScopedCritical.h>
+
+#include <Kernel/Arch/Processor.h>
+
+namespace Kernel {
+
+ScopedCritical::ScopedCritical()
+{
+    enter();
+}
+
+ScopedCritical::~ScopedCritical()
+{
+    if (m_valid)
+        leave();
+}
+
+ScopedCritical::ScopedCritical(ScopedCritical&& from)
+    : m_valid(exchange(from.m_valid, false))
+{
+}
+
+ScopedCritical& ScopedCritical::operator=(ScopedCritical&& from)
+{
+    if (&from != this) {
+        m_valid = exchange(from.m_valid, false);
+    }
+    return *this;
+}
+
+void ScopedCritical::leave()
+{
+    VERIFY(m_valid);
+    m_valid = false;
+    Processor::leave_critical();
+}
+
+void ScopedCritical::enter()
+{
+    VERIFY(!m_valid);
+    m_valid = true;
+    Processor::enter_critical();
+}
+
+}