Refactor prototype initialisation

The function do_create_interface_objects is removed in favour of 4 functions:
create_callback_interface_object, create_interface_prototype_object,
create_noncallback_interface_object and create_named_constructors.

While this increases the amount of codegen'd code, this greatly improves the
readability of the code involved in this part of DOM, instead of having one function
doing 4 different things. We can always find a more adequate abstraction later.

NativeProperties and everything related to the interface objects have been removed
from the utils module.

components/script/dom/bindings/mod.rs

@@ -138,6 +138,7 @@ pub mod conversions;
 pub mod error;
 pub mod global;
 pub mod inheritance;
+pub mod interface;
 pub mod js;
 pub mod num;
 pub mod proxyhandler;