These allow for a given block to be wrapped in helper methods or metadata and allow for more control over the current container and method before the code is run.
This allows for failover for missing partials as well as limited templating ability through the `{{> @partial-block }}` partial special case.
Partial fix for #1018
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.
Avoids parsing against SubExpressions and instead inlines the content that a subexpression otherwise would have. This can still be based via duck typing so should not add much overhead to the compiler.
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.
kpdecker