[DEPRECATE beta] Deprecate context switching form of {{with}}.

From the 2.0 RFC:

---

## More Consistent Handlebars Scope

In today's Ember, the `each` and `with` helpers come in two flavors: a "context-switching" flavor and a "named-parameter" flavor.

```handlebars
{{#each post in posts}}
  {{!-- the context in here is the same as the outside context,
        and `post` references the current iteration --}}
{{/each}}

{{#each posts}}
  {{!-- the context in here has shifted to the individual post.
        the outer context is no longer accessible --}}
{{/each}}
```

This has proven to be one of the more confusing parts of the Ember templating system. It is also not clear to beginners which to use, and when they choose the context-shifting form, they lose access to values in the outer context that may be important.

Because the helper itself offers no clue about the context-shifting behavior, it is easy (even for more experienced Ember developers) to get confused when skimming a template about which object a value refers to.

In Ember 1.10, we will deprecate the context-shifting forms of `#each` and `#with` in favor of the named-parameter forms.

### Transition Plan

To transition your code to the new syntax, you can change templates that look like this:

```hbs
{{#each people}}
  <p>{{firstName}} {{lastName}}</p>
  <p>{{address}}</p>
{{/each}}
```

with:

```hbs
{{#each people as |person|}}
  <p>{{person.firstName}} {{person.lastName}}</p>
  <p>{{person.address}}</p>
{{/each}}
```

**We plan to deprecate support for the context-shifting helpers in Ember 1.9 and remove support in Ember 2.0.** This change should be entirely mechanical.