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/KString.h

@@ -0,0 +1,131 @@
+/*
+ * 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 {
+    AK_MAKE_NONCOPYABLE(KString);
+    AK_MAKE_NONMOVABLE(KString);
+
+public:
+    [[nodiscard]] static ErrorOr<NonnullOwnPtr<KString>> try_create_uninitialized(size_t, char*&);
+    [[nodiscard]] static NonnullOwnPtr<KString> must_create_uninitialized(size_t, char*&);
+    [[nodiscard]] static ErrorOr<NonnullOwnPtr<KString>> try_create(StringView);
+    [[nodiscard]] static NonnullOwnPtr<KString> must_create(StringView);
+
+    [[nodiscard]] static ErrorOr<NonnullOwnPtr<KString>> vformatted(StringView fmtstr, AK::TypeErasedFormatParams&);
+
+    template<typename... Parameters>
+    [[nodiscard]] static ErrorOr<NonnullOwnPtr<KString>> formatted(CheckedFormatString<Parameters...>&& fmtstr, Parameters const&... parameters)
+    {
+        AK::VariadicFormatParams<AK::AllowDebugOnlyFormatters::No, Parameters...> variadic_format_parameters { parameters... };
+        return vformatted(fmtstr.view(), variadic_format_parameters);
+    }
+
+    [[nodiscard]] static ErrorOr<NonnullOwnPtr<KString>> number(Arithmetic auto value)
+    {
+        return formatted("{}", value);
+    }
+
+    void operator delete(void*);
+
+    ErrorOr<NonnullOwnPtr<KString>> try_clone() const;
+
+    [[nodiscard]] bool is_empty() const { return m_length == 0; }
+    [[nodiscard]] size_t length() const { return m_length; }
+    [[nodiscard]] char const* characters() const { return m_characters; }
+    [[nodiscard]] StringView view() const { return { characters(), length() }; }
+    [[nodiscard]] ReadonlyBytes bytes() 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> {
+    ErrorOr<void> format(FormatBuilder& builder, Kernel::KString const& value)
+    {
+        return Formatter<StringView>::format(builder, value.view());
+    }
+};
+
+template<>
+struct Formatter<OwnPtr<Kernel::KString>> : Formatter<StringView> {
+    ErrorOr<void> format(FormatBuilder& builder, OwnPtr<Kernel::KString> const& value)
+    {
+        if (value)
+            return Formatter<StringView>::format(builder, value->view());
+        return Formatter<StringView>::format(builder, "[out of memory]"sv);
+    }
+};
+
+template<>
+struct Formatter<NonnullOwnPtr<Kernel::KString>> : Formatter<StringView> {
+    ErrorOr<void> format(FormatBuilder& builder, NonnullOwnPtr<Kernel::KString> const& value)
+    {
+        return Formatter<StringView>::format(builder, value->view());
+    }
+};
+
+template<>
+struct Traits<NonnullOwnPtr<Kernel::KString>> : public GenericTraits<NonnullOwnPtr<Kernel::KString>> {
+    using PeekType = Kernel::KString*;
+    using ConstPeekType = Kernel::KString const*;
+    static unsigned hash(NonnullOwnPtr<Kernel::KString> const& p) { return string_hash(p->characters(), p->length()); }
+    static bool equals(NonnullOwnPtr<Kernel::KString> const& a, NonnullOwnPtr<Kernel::KString> const& b) { return a->view() == b->view(); }
+    static bool equals(StringView a, NonnullOwnPtr<Kernel::KString> const& b) { return a == b->view(); }
+};
+
+template<>
+struct Traits<OwnPtr<Kernel::KString>> : public GenericTraits<OwnPtr<Kernel::KString>> {
+    using PeekType = Kernel::KString*;
+    using ConstPeekType = Kernel::KString const*;
+    static unsigned hash(OwnPtr<Kernel::KString> const& p)
+    {
+        if (!p)
+            return ptr_hash(nullptr);
+        return string_hash(p->characters(), p->length());
+    }
+    static bool equals(OwnPtr<Kernel::KString> const& a, OwnPtr<Kernel::KString> const& b)
+    {
+        if (!a || !b)
+            return a.ptr() == b.ptr();
+        if (a == b)
+            return true;
+
+        return a->view() == b->view();
+    }
+    static bool equals(StringView a, OwnPtr<Kernel::KString> const& b)
+    {
+        if (!b)
+            return a.is_null();
+        return a == b->view();
+    }
+};
+
+namespace Detail {
+template<>
+inline constexpr bool IsHashCompatible<StringView, NonnullOwnPtr<Kernel::KString>> = true;
+template<>
+inline constexpr bool IsHashCompatible<StringView, OwnPtr<Kernel::KString>> = true;
+}
+
+}