`add` costs ten cycles. `push_back` isn't actually any better, but bury_the_dead can get pretty bad, especially for large arrays (like the bullets collections). also replacing the kill loop structure with the `strip` call removes a massive amount of code repetition that's costing me a lot of tokens. I think the final result is _probably_ actually slower because of function call overhead per iteration except when there are collisions on many frames; hopefully the headroom bought by the bucket collider is enough because I'm definitely going to need the tokens.
