Re: [slightlyoff/ServiceWorker] Eliminating SW startup latency for common case (#920)

A route-based sketch:

```js
self.addEventListener('install', event => {
  event.waitUntil(
    // populate caches
  );

  event.declareRoute("/", new FallbackSources(
    new CachesSource(),
    new FetchSource(),
    new FetchEventSource()
  ));
});
```

This API is up for debate wayyyyyy beyond the naming, and I still think we need better before we'd proceed with this. But here's how the above sketch could work:

**`installEvent.declareRoute(requestDescriptor, action, opts)`**

* `requestDescriptor` - which requests this route this should handle. Should be able to handle simple urls, but also a more complex form that could describe the origin, the path, the request mode etc etc. This could be a prefix by default.
* `action` - where to get the response from.
* `opts.fireFetchEvent = "no"` - (`"yes"`/`"no"`/`"passive"`) - should the fetch event trigger after the action has resolved? The response, if any, will be available in `fetchEvent.routeResponse`. This allows JS to handle the response either directly or passively.

**`new FallbackSources(…sources)`**

Attempt each source in series until an acceptable response is found. Alternatives could be `new AnySource(…sources)` which races sources until one provides an acceptable response.

**`new CachesSource(opts)`**

Look for a match in the cache. `opts` can handle the request to match on, which would be the current request by default, but could be set to something static (for getting a fallback page from the cache). Other options would cover specific caches, and things like `ignoreSearch`.

**`new FetchSource(opts)`**

Try to get a response from the network. `opts` could include the request to fetch, which would be the current request by default. `opts` could also include things like timeouts and such.

**`new FetchEventSource()`**

Defer to the fetch event.

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