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
- [x] fix places where we were using registerObserver / render queue
- [x] fix stream binding teardown
- [x] ensure streams suspend correctly during the sync queue
- [x] add stream binding tests

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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 (done by twokul in a branch)
- Refactor `ViewHelper.*` methods
- Refactor `viewHelper.call` calls in various helpers
- Don't rerender bind views when context changes