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.cpp

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Bytecode/IdentifierTable.h>
+
+namespace JS::Bytecode {
+
+IdentifierTableIndex IdentifierTable::insert(FlyString string)
+{
+    for (size_t i = 0; i < m_identifiers.size(); i++) {
+        if (m_identifiers[i] == string)
+            return i;
+    }
+    m_identifiers.append(move(string));
+    return m_identifiers.size() - 1;
+}
+
+FlyString const& IdentifierTable::get(IdentifierTableIndex index) const
+{
+    return m_identifiers[index.value()];
+}
+
+void IdentifierTable::dump() const
+{
+    outln("Identifier Table:");
+    for (size_t i = 0; i < m_identifiers.size(); i++)
+        outln("{}: {}", i, m_identifiers[i]);
+}
+
+}