LibJS: Add a separate "identifier table" to bytecode executables

This is a specialized string table for storing identifiers only.
Identifiers are always FlyStrings, which makes many common operations
faster by allowing O(1) comparison.

Userland/Libraries/LibJS/Bytecode/IdentifierTable.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/DistinctNumeric.h>
+#include <AK/FlyString.h>
+#include <AK/Vector.h>
+
+namespace JS::Bytecode {
+
+TYPEDEF_DISTINCT_NUMERIC_GENERAL(size_t, false, true, false, false, false, false, IdentifierTableIndex);
+
+class IdentifierTable {
+    AK_MAKE_NONMOVABLE(IdentifierTable);
+    AK_MAKE_NONCOPYABLE(IdentifierTable);
+
+public:
+    IdentifierTable() = default;
+
+    IdentifierTableIndex insert(FlyString);
+    FlyString const& get(IdentifierTableIndex) const;
+    void dump() const;
+    bool is_empty() const { return m_identifiers.is_empty(); }
+
+private:
+    Vector<FlyString> m_identifiers;
+};
+
+}