Web client performance

Some different ideas for smoothing out the web client:

• The .wasm binary is around 50MB, which makes initial startup time quite sad. Gzipped is around 20MB. I can't find any option to enable this in wasm-pack, because I think people generally just configure their web servers to do compression on-the-fly if the HTTP client sends Accept-Encoding headers. S3 doesn't support this, but I think it's possible to upload a .wasm.gz, point to it from the .html, and set the Content-Encoding header properly. Just need to test this and make sure browsers understand it.
• Keep the Scenario for the current map in memory after loading it from a file. Otherwise, everytime the player resets to midnight in many gameplay modes, they have to download the file again!
• In wasm_loader::FileLoader, display a progress bar with bytes downloaded / total length. I found set_onprogresss for XmlHttpRequest, but actually, we're currently using web_sys::Request, which is different. Can we switch?