Implement the remainder of the feature started in #906.

We want to allow for helpers to provide named values that may be accessed by any child program run. The goal is to allow explicit naming of helper data fields so they can easily be accessed in nested scope, vs. arbitrary @../index references.

For example:

{{#each foos as |foo indexFoo|}}
  {{#each bars as |bar indexBar|}}
    {{indexFoo}}.{{indexBar}}. {{foo}} {{bar}}
  {{/each}}
{{/each}}