Implement WebAssembly backend

[bitwalker]

• Tooling to generate browser bundle (everything needed to run in the browser) from compiled WASM module
• Flesh out runtime support (via web-sys/js-sys)

[KronicDeth]

For browser bundle, look at wasm-pack build to see what we want to use or borrow from it.

[KronicDeth]

The Web loop for the scheduler is invoked with lumen_web::start in #206. Getting code to run in that scheduler in the background is done with lumen_web::wait::with_return_0::spawn.

All my stuff is still using wasm-pack build though, so nothing to help with the first point.