Don't change the current context if using `#each ... as |...|`

It does not make sense at all that a snippet like this:
```hbs
{{someVal}}
{{#each profiles as |profile|}}
{{profile.username}}
{{someVal}}
{{/each}}
```
changes the current context, instead of just assigning the new context (the current element of `profiles`) to a new identifier (profile).

`{{someVal}}` should be accessible inside and outside the `{{#each}}` block, instead of forcing us to do this:
```hbs
{{someVal}}
{{#each profiles as |profile|}}
{{profile.username}}
{{@root.someVal}}
{{/each}}
```

If `profiles` already exits in the current context, it should be overwritten.

The issue does not name a file or test. Reproduce the two provided Handlebars snippets, then locate the implementation and tests for #each block context and named aliases; done means someVal resolves inside and outside the block while profile remains available as the current element.

Written by the indexing model from the issue text.