Eliminate unecessary monster enumeration

[kevingranade]

Summary

SUMMARY: Performance "Avoid unecessarally enumerating monsters."

Purpose of change

This operation scales as O(n^2) in the number of monsters on the map, meaning with enough monsters it dominates all other computation.

Describe the solution

Only enumerate the monsters if we need to, which is when a monster has a need to size up other monsters.

Describe alternatives you've considered

There are potentially ways to either cache this list or make it much cheaper to produce, but I want to wait on a demonstrable need to dive into that kind of thing.

Additional context

To trigger, give the player invincibility, and debug spawn monsters with a large radius (I used 50). Radius 50 gives a bit over 10,000 monsters on the map. The symptom is single turns taking multiple seconds to resolve.