This PR replaces recursive explosions with a new spreading engine: iterative explosions.
Should behave similarly, but also be more reliable, potentially faster.
Iterative explosions also support simple-explosion-esque directional explosion sounds/shake, and will now traverse Z-levels if the explosion is strong enough.
Also changes playsound() to transmit sounds across Z-levels. Uses BYOND's 3rd coordinate on /sound to make sound sound above/below the player if they're using headphones. Removed for now, can't get Z-falloff working right.
Fixes#2199.