Rewriting multiz movement to be gooder (#2464)

Intent: implement all of the features from #2442 while also unfucking the multiz movement files to be more readable and not recurse.

To that end, this PR does the following:

All multiz travel is now arbitrated by SSfalling. This will eliminate the need for recursion without relying on timers. Timers would be a bit scary.
The call chain for multiz movement now looks like this (in terms of overwritable procs):
can_fall() (Can lead into fall_impact() & fall_collateral() if returns FALSE)
fall_through() IF current block is open space ELSE fall_impact() & fall_collateral()
Removed almost every istype(src, A) check in the movement.dm file by exercising proper parenting and call chains.
Documented and standardized the contents of multiz/movement.dm. Because this is an API we're going to be relying on very heavily, I'd like to get this right before we move on.
A lot of minor tweaks, like swapping usr over to the applicable mob, and so on.

code/modules/mob/living/living_defines.dm

@@ -46,7 +46,6 @@
 
 	var/failed_last_breath = 0 //This is used to determine if the mob failed a breath. If they did fail a brath, they will attempt to breathe each tick, otherwise just once per 4 ticks.
 	var/possession_candidate // Can be possessed by ghosts if unplayed.
-	var/z_levels_fallen = 0 //For z-level damage stacking.
 
 	var/list/stomach_contents	//This is moved here from carbon defines
 	var/composition_reagent