Triggers.projectile_detonated, Triggers.got_item, etc., should return indices of the objects that were removed, if possible

[aaronfreed]

This way, scripts that keep track of references to those objects can immediately know that those references are now invalidated and should be removed. The current way of doing this is a colossal glutealgia*, and streamlining it would prevent a lot of inefficient code from needing to be run on the Lua side for such scripts.

*i.e., a massive pain in the ass