core: Remove LoaderHandle when load is done

[Aaron1011]

In almost all cases, we were letting completed LoaderHandles accumulate, leaking memory and wasting time in preload_tick

[Dinnerbone]

Sorry, maybe reviewed too quickly.

I notice that some places have the potential to return early with ? before getting to this line. Is that going to be an issue?

Is there another way we can do this?

[kmeisthax]

I'm wondering if we can abuse RAII/Drop to make loaders close themselves regardless of whether or not they exit normally or not.