WIP HTMLBars

Build TODO:

* [ ] Refactor the build pipeline to pull in HTMLBars via ES6.
* [ ] Have the ember-htmlbars test helper come out of htmlbars via ES6 (is this a new htmlbars package that doesn't ship in the main build?).
* [ ] Ensure the ember-htmlbars package in Ember is not included in the Ember build.
* [ ] simple-html-tokenizer should not be needed in Ember (it comes with HTMLBars already).

TODO:

* [ ] `view` helper must be implemented, tested.
* [ ] `component` helper must be implemented, tested (with yielding to a string and fragment).
* [ ] Move Handlebars views used in HTMLBars into ember-views. Rename, refactor along the way.
* [ ] Upstream commit to HTMLBars for to emit blank array types.
* [ ] Upstream commit to HTMLBars to fix the equalHTML helper on bare text nodes.
* [ ] (unrelated, but the refactor of buffer should go into beta. Must confirm it fixes an issue with rerender of TRs)
* [ ] Refactor the  `Object.create` out of `views/view`.
* [ ] Add a feature flag around the template arity check in `views/view`.
* [ ] Rebase shizzit (after some other things get fixed)
* [ ] Rebase on top of Handlebars 2 (requires upstream fix for Handlebars 2)