Lane-changing

Ah, this bundle of joy.

Steps:

decide if LCing can ever fail (or if cars will just stop and wait to merge)
query to know when is it safe (similar to spawn checking)
the state to execute it (and assert it completes in time)
the modifications to lookahead
change pathfinding to understand new movements
- how many LCs can be done on one road?
- how to render it? = change what turns are initially created