Move bind-attr tests to own module
Create an ember-htmlbars package and bring in all the deps. Again.
Initial pass at porting helpers to HTMLBars

HTMLBars has a slightly different syntax for helpers than Handlebars.
All helpers are called by a hook, which performs several tasks to ensure
consistency and predictability.

1. All helpers have the same arguments: params, options, env
2. The params list is a set of streams
3. The options data is specific to this helper call (isEscaped etc)
4. The env is safe for re-use. It can be passed to a child template for
instance.

In order for most helpers to work, a few parts of Ember needed to be
morph aware. In old Ember, a child view being added from a helper would
call pushChildView and cause a morph to be added to the parent's buffer.
In modern Ember, the HTMLBars provides a morph to the helper at
options.morph, and this is attached to the view as _morph before a
child view renders.

A few helpers are implemented as part of this first pass, and show the
start of a way forward for the others:

* `{{bind}}`
* `{{yield}}`
* `{{view}}`
* `{{if}}` and `{{unless}}`
* `{{with}}`

Additionally, HTMLBars helpers can be registerd with a method.
Merge pull request #9358 from mixonic/htmlbars

HTMLBars: Initial Helper Integration
Merge pull request #9526 from mmun/extract-bind-attr-tests

Move bind-attr tests to own module
Deprecate context switching for of {{each}}.