Introducing ExtensionIRNode and an implementation of templates based on
the new feature set.
Now TemplateIRNode is-a ExtensionIRNode. It's implemented using just
extensibility and isn't part of the standard razor codegen. I'm adding it
to the RazorEngine so that it's still there by default.
I've also included a pattern for visitors to special case
ExtensionIRNode-derived classes that they know about. This requires a
little bit of boilerplate but makes it easy to traverse just the nodes you
care about while keeping the set of nodes open.
For now the general codegen feature still hasn't had a refactor, but this
opens things up for us to start finishing things like MVC's @inject
directive.
