Potential fix for anything relying on self to be single.

NEEDS COMMENT: please read

As reported by egofree:

> Here is a test case : https://dl.dropboxusercontent.com/u/29802501/object_deactivate.egm
> 
> When the project starts, you have a black square which goes to the right and collides with a red square. In the red square object, i've a collision event with the following line : instance_deactivate_object(self); As you can see, when it collides with the red square, all red squares disappear. This is not the case in GM studio. (You can export the project as a gmx project and test it with GM Studio).

From my tests, this will also cause ```with(self) {}``` to fire for every instance of that object type.

I tracked this down to fetch_inst_iter_by_int()  ---it seems that we should really be returning an iterator that wraps the instance inside instance_event_iterator, rather than the iterator itself (which contains next/prev pointers to the siblings in the event hierarchy). I am not 100% sure this is the right thing to do, hence the "potential" fix. This patch does fix the behavior egofree reported, though.