More features for externs

As discussed with various people at WWX we should look into adding some more features for extern classes to make native integration easier. The two main ones are operator overloading and custom array access.

I think this should be easy because we don't have to worry about the run-time behavior. As long as our selection algorithm is wide enough to match the native semantics, everything else should be taken care of by the run-time already.

Start by reviewing the compiler's extern-class handling and the selection algorithm mentioned in the issue. Determine the scope needed for operator overloading and custom array access, and verify that both match the intended native semantics without requiring runtime changes.

Written by the indexing model from the issue text.