* On the web, load different maps by making asynchronous HTTP calls. #21
This is a very strange, specializd approach; it's unclear how it'll
generalize to reading scenarios and prebaked results. Ideally we could
call abstutil::read_binary as we currently do and somehow hide this async
trickery underneath, but I'm not sure how yet. In the meantime, this
moves us forward with the hack well-contained.
Next steps for web: stop bundling in all of data/system in the .wasm,
now that we can load from HTTP.
