Default backend should generate TemplateHaskell and write that to a file

Although the current practice of simply concatenating Strings and dumping the result in a .hs file is simple, I think the general plan should be to do all the staging as part of (Typed?) Template Haskell meta programming. A very reasonable step in-between is to generate a TH AST in the backend which is subsequently pretty-printed to generate the .hs file.