LibJS: Add an optimization to avoid needless arguments object creation

This gives FunctionNode a "might need arguments object" boolean flag and
sets it based on the simplest possible heuristic for this: if we
encounter an identifier called "arguments" or "eval" up to the next
(nested) function declaration or expression, we won't need an arguments
object. Otherwise, we *might* need one - the final decision is made in
the FunctionDeclarationInstantiation AO.

Now, this is obviously not perfect. Even if you avoid eval, something
like `foo.arguments` will still trigger a false positive - but it's a
start and already massively cuts down on needlessly allocated objects,
especially in real-world code that is often minified, and so a full
"arguments" identifier will be an actual arguments object more often
than not.

To illustrate the actual impact of this change, here's the number of
allocated arguments objects during a full test-js run:

Before:
- Unmapped arguments objects: 78765
- Mapped arguments objects: 2455

After:
- Unmapped arguments objects: 18
- Mapped arguments objects: 37

This results in a ~5% speedup of test-js on my Linux host machine, and
about 3.5% on i686 Serenity in QEMU (warm runs, average of 5).

The following microbenchmark (calling an empty function 1M times) runs
25% faster on Linux and 45% on Serenity:

    function foo() {}
    for (var i = 0; i < 1_000_000; ++i)
        foo();

test262 reports no changes in either direction, apart from a speedup :^)

Userland/Libraries/LibJS/Runtime/GeneratorFunctionConstructor.cpp

@@ -62,7 +62,7 @@ Value GeneratorFunctionConstructor::construct(FunctionObject& new_target)
             block.dump(executable);
     }
 
-    return ECMAScriptFunctionObject::create(global_object(), function->name(), function->body(), function->parameters(), function->function_length(), vm().lexical_environment(), FunctionKind::Generator, function->is_strict_mode(), false);
+    return ECMAScriptFunctionObject::create(global_object(), function->name(), function->body(), function->parameters(), function->function_length(), vm().lexical_environment(), FunctionKind::Generator, function->is_strict_mode(), function->might_need_arguments_object());
 }
 
 }