refactor codegen file to use template2ast

[vshesh]

Shouldn't be dealing with the same if cases in every spot. Rather, since template2ast is a standard ast, I can operate directly on that.

The bad thing about this is that I've now made it into an ast, which i didn't want to do initially. But I'd much rather have one place where I change what the representation of the elements looks like rather than having to do it for every language.