Embed osm2streets in map_model

Currently the `raw_map` layer embeds osm2streets `StreetNetwork`, just adding A/B Street-specific stuff in addition (buildings, areas, transit routes, and things like crosswalks & modal filters that aren't yet in osm2streets). But then the `map_model` layer, which is used by the traffic sim and all the apps, has its own representation for roads and intersections, and all the osm2streets stuff is gone. This was OK to start, but it's starting to get a bit tricky:

- A/B Street edits lane specs and needs to regenerate geometry and movements, so we have an adapter kind of hack around it now
- Rendering code is starting to be duplicated between `map_gui` and osm2streets. And then again in #1038 for Bevy
- There are some osm2streets API concepts like `RoadEdge` that'd be useful to use over here now, but they're not easy to get at

I'd like to explore options for embedding `StreetNetwork` inside `map_model::Map` too. Reasons to be wary of this:

- Keeping serialized file size small and loading times quick for large maps is important, potentially at the expense of code readability / API niceness
- the API surface of `Lane`, `Road`, `Intersection` is huge and messy
- Keeping some indirection / wrapper between osm2streets and here is a good idea for evolving the two codebases at different speeds

Another vague idea is to embed `osm2streets::{Road, Intersection}` structs inside of A/B Street equivalents, and keep a few extra fields over on this side

CC @BudgieInWA