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.
Blobs now randomly pick a random turf without obstructions in maintenance, instead of from a pre-determined list of locations.
Blobs also now log their spawn location, which admins can use to jump to it.
Space vines and nuclear discs have been updated to use the same mechanics.
