LibJS: Move native objects towards two-pass construction

To make sure that everything is set up correctly in objects before we
start adding properties to them, we split cell allocation into 3 steps:

1. Allocate a cell of appropriate size from the Heap
2. Call the C++ constructor on the cell
3. Call initialize() on the constructed object

The job of initialize() is to define all the initial properties.
Doing it in a second pass guarantees that the Object has a valid Shape
and can find its own GlobalObject.

Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -41,7 +41,7 @@ PrimitiveString::~PrimitiveString()
 
 PrimitiveString* js_string(Heap& heap, String string)
 {
-    return heap.allocate<PrimitiveString>(move(string));
+    return heap.allocate<PrimitiveString>(heap.interpreter().global_object(), move(string));
 }
 
 PrimitiveString* js_string(Interpreter& interpreter, String string)