Representation of a #parse directive in the parse tree.

A #parse (<primary>) directive does nothing until the template is evaluated. Then the <primary> should evaluate to a string, which will be used to derive a nested Template. If the string was seen before (in this evaluation, or an earlier evaluation) then the Template will be retrieved from the parseCache. Otherwise we will get a Reader from the ResourceOpener and parse its contents to produce a new Template, which we will record in the parseCache. Either way, we will execute the nested Template, which means adding its macros to the EvaluationContext and evaluating it to add text to the final output.