Ghost biking

Right now, bikes act just like cars in the simulation -- they queue behind each other, and vehicles queue behind them. There is [practically no lane-changing](https://a-b-street.github.io/docs/tech/trafficsim/discrete_event/index.html#lane-changing) in the simulation, so lots of things wind up quite unrealistic:

- cars getting stuck behind slow bikes
- bikes not overtaking each other -- this'd become relevant if we introduced e-bikes
- gridlock happening with lots of bikes
- bikes spacing out way too much when stopped (#187)

Improving the lane-changing model and attempting the same level of realism it has currently is... unlikely to happen in the foreseeable future. Also, treating lanes as discrete objects ignores reality. Lots of roads don't have a marked center-line and designated lanes and shoulders; sometimes there's room for a car to squeeze past a bike on the shoulder, sometimes they dip into oncoming traffic a bit. Until we have real road width data, there's not really a way we could even hope to model all this variation.

Based on recent conversations with @Robinlovelace about the goals for A/B Street's traffic simulation and the amount of realism that's useful, I think we've decided it'd be nice to try something much simpler. Pedestrians can "ghost" through each other; let's treat bikes the same way. This means:

- cars will invisibly pass through a bike
- a faster bike will pass through a slower bike
- at an intersection, all the bikes will clump to the front, ghosting through each other and through any cars that're in front of them

Rendering-wise, this will look quite weird -- probably have to make sure the Z-order is consistent between frames, at least.

Maybe a slightly more complex, but useful, model is to note when a bike is overlapping part of a lane with any other vehicle, and 1) adjust the rendering somehow, and 2) adjust the speed of both vehicles somehow.