AK: Implement FlyString for the new String class

This implements a FlyString that will de-duplicate String instances. The
FlyString will store the raw encoded data of the String instance: If the
String is a short string, FlyString holds the String::ShortString bytes;
otherwise FlyString holds a pointer to the Detail::StringData.

FlyString itself does not know about String's storage or how to refcount
its Detail::StringData. It defers to String to implement these details.

AK/FlyString.cpp

@@ -0,0 +1,169 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/FlyString.h>
+#include <AK/HashMap.h>
+#include <AK/Singleton.h>
+#include <AK/StringView.h>
+#include <AK/Utf8View.h>
+
+namespace AK {
+
+static auto& all_fly_strings()
+{
+    static Singleton<HashMap<StringView, uintptr_t>> table;
+    return *table;
+}
+
+FlyString::FlyString()
+    : m_data(String {}.to_fly_string_data({}))
+{
+}
+
+FlyString::~FlyString()
+{
+    String::unref_fly_string_data({}, m_data);
+}
+
+ErrorOr<FlyString> FlyString::from_utf8(StringView string)
+{
+    return FlyString { TRY(String::from_utf8(string)) };
+}
+
+FlyString::FlyString(String const& string)
+{
+    if (string.is_short_string()) {
+        m_data = string.to_fly_string_data({});
+        return;
+    }
+
+    auto it = all_fly_strings().find(string.bytes_as_string_view());
+    if (it == all_fly_strings().end()) {
+        m_data = string.to_fly_string_data({});
+
+        all_fly_strings().set(string.bytes_as_string_view(), m_data);
+        string.did_create_fly_string({});
+    } else {
+        m_data = it->value;
+    }
+
+    String::ref_fly_string_data({}, m_data);
+}
+
+FlyString::FlyString(FlyString const& other)
+    : m_data(other.m_data)
+{
+    String::ref_fly_string_data({}, m_data);
+}
+
+FlyString& FlyString::operator=(FlyString const& other)
+{
+    if (this != &other) {
+        m_data = other.m_data;
+        String::ref_fly_string_data({}, m_data);
+    }
+
+    return *this;
+}
+
+FlyString::FlyString(FlyString&& other)
+    : m_data(other.m_data)
+{
+    other.m_data = String {}.to_fly_string_data({});
+}
+
+FlyString& FlyString::operator=(FlyString&& other)
+{
+    m_data = other.m_data;
+    other.m_data = String {}.to_fly_string_data({});
+
+    return *this;
+}
+
+bool FlyString::is_empty() const
+{
+    return bytes_as_string_view().is_empty();
+}
+
+unsigned FlyString::hash() const
+{
+    return bytes_as_string_view().hash();
+}
+
+FlyString::operator String() const
+{
+    return to_string();
+}
+
+String FlyString::to_string() const
+{
+    return String::fly_string_data_to_string({}, m_data);
+}
+
+Utf8View FlyString::code_points() const
+{
+    return Utf8View { bytes_as_string_view() };
+}
+
+ReadonlyBytes FlyString::bytes() const
+{
+    return bytes_as_string_view().bytes();
+}
+
+StringView FlyString::bytes_as_string_view() const
+{
+    return String::fly_string_data_to_string_view({}, m_data);
+}
+
+bool FlyString::operator==(FlyString const& other) const
+{
+    return m_data == other.m_data;
+}
+
+bool FlyString::operator==(String const& other) const
+{
+    if (m_data == other.to_fly_string_data({}))
+        return true;
+
+    return bytes_as_string_view() == other.bytes_as_string_view();
+}
+
+bool FlyString::operator==(StringView string) const
+{
+    return bytes_as_string_view() == string;
+}
+
+bool FlyString::operator==(char const* string) const
+{
+    return bytes_as_string_view() == string;
+}
+
+void FlyString::did_destroy_fly_string_data(Badge<Detail::StringData>, StringView string_data)
+{
+    all_fly_strings().remove(string_data);
+}
+
+uintptr_t FlyString::data(Badge<String>) const
+{
+    return m_data;
+}
+
+size_t FlyString::number_of_fly_strings()
+{
+    return all_fly_strings().size();
+}
+
+unsigned Traits<FlyString>::hash(FlyString const& fly_string)
+{
+    return fly_string.bytes_as_string_view().hash();
+}
+
+ErrorOr<void> Formatter<FlyString>::format(FormatBuilder& builder, FlyString const& fly_string)
+{
+    return Formatter<StringView>::format(builder, fly_string.bytes_as_string_view());
+}
+
+}