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.cpp

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/KString.h>
+
+namespace Kernel {
+
+OwnPtr<KString> KString::try_create(StringView const& string)
+{
+    char* characters = nullptr;
+    size_t length = string.length();
+    auto new_string = KString::try_create_uninitialized(length, characters);
+    if (!new_string)
+        return {};
+    if (!string.is_empty())
+        __builtin_memcpy(characters, string.characters_without_null_termination(), length);
+    characters[length] = '\0';
+    return new_string;
+}
+
+OwnPtr<KString> KString::try_create_uninitialized(size_t length, char*& characters)
+{
+    size_t allocation_size = sizeof(KString) + (sizeof(char) * length) + sizeof(char);
+    auto* slot = kmalloc(allocation_size);
+    if (!slot)
+        return {};
+    auto* new_string = new (slot) KString(length);
+    characters = new_string->m_characters;
+    return adopt_own(*new_string);
+}
+
+OwnPtr<KString> KString::try_clone() const
+{
+    return try_create(view());
+}
+
+}