change the creature_tracker to use a list of pointers to monsters.

This allows infestation/removal of monsters without changing the references that `Creature_tracker::find` / `game::zombie` returns.
Revert using temporary copy copy of monster during its death function.

The previous commit removed the possible reallocation of the monster object. One can now safely use the monster object until it is actually removed from the creature tracker. So there is no need for the temporary copy.