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.cpp

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