LibJS: Some optimizations for ObjectExpression

- move() the property map when constructing ObjectExpression instead of
  making a copy.
- Use key+value iterators to traverse the property map in the execute()
  and dump() functions.

Libraries/LibJS/AST.h

@@ -574,7 +574,7 @@ private:
 class ObjectExpression : public Expression {
 public:
     ObjectExpression(HashMap<String, NonnullRefPtr<Expression>> properties = {})
-        : m_properties(properties)
+        : m_properties(move(properties))
     {
     }