Rename thinkers.iterate to mobjs.iterate, kill the "mobjs" arg (though you...

Rename thinkers.iterate to mobjs.iterate, kill the "mobjs" arg (though you still need the parentheses). Done a bit messily/lazily, I'll clean up if we can confirm this works

I do wonder whether this library can be extended for all of thlist's lists... but for now it's mobj-only, since we only have the ability to mess with mobjs anyway.