This allows us to avoid creating unnecessary AST nodes and avoids things like isHelper.
Side effect of these changes is that @data functions can now have data parameters passed to them.
Rather than keeping state in the AST, which requires some gymnastics, we create a separate visitor flow which does the top down iteration necessary to calculate all of the state needed for proper whitespace control evaluation.
Allows users to chain multiple helpers together using their inverse callbacks. I.e.
```
{{#if foo}}
{{else if bar}}
{{else}}
{{/if}}
```
The control flow here effectively causes the helpers to be nested. The above is actually syntactic sugar for this:
```
{{#if foo}}
{{else}}
{{#if bar}}
{{else}}
{{/if}}
{{/if}}
```
Any helper may be used in this manner, the only requirement is they support normal calls and inverse calls.
Introduces a breaking change in that `{{else foo}}` may no longer be used as a root level operator. Instead `{{^foo}}` must be used.
Fixes#72.
We already have to track these behaviors for the standalone parsing and rather than having two whitespace pruning implementations this moves all of the behavior into one place.
Fixes#852
kpdecker
Martin Muñoz