Groupby in clang/grappalang

- [x] clang
- [x] test clang
- [x] adapt to updates from #209 
- [x] grappalang
- [x] test grappalang (#228 merged)

corresponds to Grappa https://github.com/uwsampa/grappa/commit/0be42b6168f087262160b07e037493e1a390fc3f
@bmyerz: fyi, @7andrew7 and I got rid of these stringify tests in the forthcoming PR #248. They just don't really test anything super useful.