Integrate with Cloudflare Workers

[armanbilge]

I started this in #4. It's an old and big PR, so I'll probably divvy it up.

Assorted notes:

• There are both workers, and also these DurableObject things which I think get deployed independently. DurableObjects are hard/complex, and better for a follow-up.
• Workers can now be written as ES6 modules. https://blog.cloudflare.com/workers-javascript-modules/
  Enable Closure when emitting an ES module. scala-js/scala-js#3893
• There is no local runtime for workers. They run in a custom V8 based runtime with Web Worker style APIs. So testing is going to be annoying. Aha, there is https://github.com/cloudflare/miniflare now :)
• An sbt-plugin to deploy a worker should be easy, just uploading to an endpoint.
• The JS sources for a worker are restricted to 1 mb gzipped. Looking at you, http4s MimeDB 👀