Kernel: Add KString, a single-owner string with OOM failure exposion

This is a simple string class for use in the kernel. It encapsulates
a length + character array in a single-allocation object.

Main differences from AK::String:

- Single-owner (no reference counting.)
- Allocation failures are exposed, not hidden.

The basic idea is to allow better and more precise string management
in the kernel.

Kernel/KString.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Format.h>
+#include <AK/OwnPtr.h>
+
+namespace Kernel {
+
+class KString {
+public:
+    static OwnPtr<KString> try_create_uninitialized(size_t, char*&);
+    static OwnPtr<KString> try_create(StringView const&);
+
+    OwnPtr<KString> try_clone() const;
+
+    bool is_empty() const { return m_length == 0; }
+    size_t length() const { return m_length; }
+    char const* characters() const { return m_characters; }
+    StringView view() const { return { characters(), length() }; }
+
+private:
+    explicit KString(size_t length)
+        : m_length(length)
+    {
+    }
+
+    size_t m_length { 0 };
+    char m_characters[0];
+};
+
+}
+
+namespace AK {
+
+template<>
+struct Formatter<Kernel::KString> : Formatter<StringView> {
+    void format(FormatBuilder& builder, Kernel::KString const& value)
+    {
+        Formatter<StringView>::format(builder, value.characters());
+    }
+};
+
+}