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.
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