REF: use fused types for groupby_helper
There will be some nice cleanups we can do after we bump cython to 0.30 (which hasnt come out yet).

Also I think there is some na-checking code that we can share between the various fused-types functions after this.
>  I'd still prefer to keep as is and just add the uint template generation as required until 0.30 gets released

Since we're probably going to wait for a while after 0.30 is released before bumping ourselves, that is longer than I'd prefer.

As for using uint template generation in the interim, I did that before deciding that I am much more likely to screw up using tempita than I am using fused types. thanks @jbrockmendel 

we cannot wait for deps, who knows when they land