Streamify template bindings

- [x] Rewrite views/helpers not to use View#registerObserver / handlebarsGet / normalizePath
  - [x] class name bindings
  - [x] link-to
  - [x] makeBoundHelper
  - [x] simpleBind
  - [x] bind
- [x] Streamify keywords
  - [x] LazyValueBinding supports a (keyword) stream as source object
  - [x] Move keywords to view._keywords

(Credits to @ebryn for LazyValue and other code from the htmlbars branch)
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