Leaking views

In any of the following cases:
```
{{#collection}}
  <div>Foo</div>
  {{view innerView}}
{{/collection}}
```
or
```
{{#collection}}
  <div>Foo</div>
  {{view "path/to/named-inner-view"}}
{{/collection}}
```
when any model in the collection is changed, the corresponding item template is getting re-rendered, but the inner view instances aren't getting destroyed before re-rendering which causes a memory leak.