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

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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

seems good. the only thing to worry about is that the callback gets passed the stream as its first argument so that it can be used with multiple streams. this is fine for rerender because it doesn't take an argument.