More sexps

I'm still working on making a nicer sexp-based represention of Futhark. Yesterday, Troels made a handful of improvements to my PR, and today I've continued the work a bit. Unfortunately, there are still some issues with sexp-grammar that need to be fixed before we can merge anything.

It looks better now! We still need to wait for nix to update their haskell packages though. And for some reason, stack needs this?