Streamify template bindings

Necessary to merge:

- [x] stream chains should be composed of KeyStreams
- [x] make view context a stream with keyword resolution layered into it
- [x] fix itemClassBinding
- [ ] fix stream & binding teardown
- [ ] fix places where we were using registerObserver / render queue

---

Other nice things that should be done:

- Context streams should be chained off of parent context streams.
- Defer Stream#notify to the render queue?
- Make `this` in a helper be the view that is invoking the helper
- Refactor `ViewHelper.*` methods
- Refactor `viewHelper.call` calls in various helpers
- Don't rerender bind views when context changes

```
function callback(stream) { }

stream1 = new Stream();
stream2 = new Stream();
stream1.subscribe(callback);
stream2.subscribe(callback);
```

Just saying that that callback signature matters. gotta be careful. rerender doesn't take any arguments so it works fine