LibJS: Implement correct object property ordering

This commit introduces a way to get an object's own properties in the
correct order. The "correct order" for JS object properties is first all
array-like index properties (numeric keys) sorted by insertion order,
followed by all string properties sorted by insertion order.

Objects also now print correctly in the repl! Before this commit:

courage ~/js-tests $ js
> ({ foo: 1, bar: 2, baz: 3 })
{ bar: 2, foo: 1, baz: 3 }

After:

courage ~/js-tests $ js
> ({ foo: 1, bar: 2, baz: 3 })
{ foo: 1, bar: 2, baz: 3 }

Libraries/LibJS/Runtime/Cell.cpp

@@ -56,6 +56,11 @@ Interpreter& Cell::interpreter()
     return heap().interpreter();
 }
 
+Interpreter& Cell::interpreter() const
+{
+    return heap().interpreter();
+}
+
 const LogStream& operator<<(const LogStream& stream, const Cell* cell)
 {
     if (!cell)

Libraries/LibJS/Runtime/Cell.h

@@ -60,6 +60,7 @@ public:
 
     Heap& heap() const;
     Interpreter& interpreter();
+    Interpreter& interpreter() const;
 
 protected:
     Cell() {}

Libraries/LibJS/Runtime/Object.cpp

@@ -34,6 +34,7 @@
 #include <LibJS/Runtime/NativeProperty.h>
 #include <LibJS/Runtime/Object.h>
 #include <LibJS/Runtime/Shape.h>
+#include <LibJS/Runtime/StringObject.h>
 #include <LibJS/Runtime/Value.h>
 
 namespace JS {

Libraries/LibJS/Runtime/ObjectConstructor.cpp

@@ -76,7 +76,7 @@ Value ObjectConstructor::get_own_property_names(Interpreter& interpreter)
             result->elements().append(js_string(interpreter, String::number(i)));
     }
 
-    for (auto& it : object->shape().property_table()) {
+    for (auto& it : object->shape().property_table_ordered()) {
         result->elements().append(js_string(interpreter, it.key));
     }
     return result;

Libraries/LibJS/Runtime/Shape.cpp

@@ -117,6 +117,18 @@ size_t Shape::property_count() const
     return property_table().size();
 }
 
+Vector<Shape::Property> Shape::property_table_ordered() const
+{
+    auto vec = Vector<Shape::Property>();
+    vec.resize(property_table().size());
+
+    for (auto& it : property_table()) {
+        vec[it.value.offset] = { it.key, it.value };
+    }
+
+    return vec;
+}
+
 void Shape::ensure_property_table() const
 {
     if (m_property_table)

Libraries/LibJS/Runtime/Shape.h

@@ -88,6 +88,13 @@ public:
     const HashMap<FlyString, PropertyMetadata>& property_table() const;
     size_t property_count() const;
 
+    struct Property {
+        FlyString key;
+        PropertyMetadata value;
+    };
+
+    Vector<Property> property_table_ordered() const;
+
     void set_prototype_without_transition(Object* new_prototype) { m_prototype = new_prototype; }
 
     void remove_property_from_unique_shape(const FlyString&, size_t offset);