LibJS: Store strings in a string table

Instead of using Strings in the bytecode ops this adds a global string
table to the Executable struct which individual operations can refer
to using indices. This brings bytecode ops one step closer to being
pointer free.

Userland/Libraries/LibJS/Bytecode/StringTable.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/DistinctNumeric.h>
+#include <AK/String.h>
+#include <AK/Vector.h>
+
+namespace JS::Bytecode {
+
+TYPEDEF_DISTINCT_NUMERIC_GENERAL(size_t, false, true, false, false, false, false, StringTableIndex);
+
+class StringTable {
+    AK_MAKE_NONMOVABLE(StringTable);
+    AK_MAKE_NONCOPYABLE(StringTable);
+
+public:
+    StringTable() = default;
+
+    StringTableIndex insert(StringView string);
+    String const& get(StringTableIndex) const;
+    void dump() const;
+    bool is_empty() const { return m_strings.is_empty(); }
+
+private:
+    Vector<String> m_strings;
+};
+
+}