killing is now done based on groups, the same way enemies already killed
the player.
these groups go on the entity's hitbox, not the entity itself.
enemies' hitboxes have all been put in "enemy_hitbox"
the arrow_projectile has a variable for the target group, so it could
easily be simply set to "player" for arrows shot by enemies
blocking is also done with groups. any hitbox with "blocks_arrow"
will block arrows, same with "blocks_sword" and "blocks_squash"
