Support subscribing to an EventSource

[RoyalIcing]

• https://www.html5rocks.com/tutorials/eventsource/basics/

const machine = start(Machine);
const eventSource = new EventSource();

machine.receiveFrom(eventSource);

const messagesKey = Symbol("messages");

function* Machine() {
  yield listenTo(eventSource, "open");
  yield on(new Map([["type", "error"], ["readyState", EventSource.CLOSED]]), Closed);

  function* Open() {
    yield listenTo(eventSource, "message");
    yield accumulate("message", messagesKey);
  }
  function* Closed() {}
  
  return function* Initial() {
    yield listenTo(eventSource, "open");
    yield on("open", Open);
  }
}

[RoyalIcing]

This is implemented. It does seem to run in an infinite loop, re-opening after it closes.