This PR replaces our usage of EventEmitter with an interface with the following shape:

interface Emitter {
  emit(type: string, event?: any): void;
  on(type: string, handler: (event?: any) => void): void;
  off(type: string, handler: (event?: any) => void): void;
}

This allows the consumer to provide their own emitter, or opt not to. For users who opt to use a small emitter like mitt, this should result in a decently smaller bundle size.

I also updated the HTTP tests to run the entire suite with an EventEmitter, with mitt, and without an event emitter at all.

Fixes #679