add uint64 support for some libgroupby funcs
cc @WillAyd I hope you agree the runtime_error thing here is easier to implement/review with fused type than it would be with tempita.
Can you add tests for these as well? Would be nice to have something parametrized in groupby > Can you add tests for these as well? Would be nice to have something parametrized in groupby

I'll take a look and see if we have tests that get at these functions directly (as opposed to via df.groupby.whatever).  I can attest that these are hit by the python code for which this adds a comment. Following this and #28934 one more obvious cleanup is to move all of groupby_helper into groupby.pyx, since we are no longer using the templating @WillAyd it looks like we don't have any dedicated testing that gets directly a the functions in this file. Any major concerns?  This is a blocker for getting at the `except Exception`s in/around _cython_agg_blocks > I think you can add to the parametrization of this test here:

@WillAyd added uint64 to this parametrization. I think a whatsnew for function support would be nice as well > I think a whatsnew for function support would be nice as well

Only user-facing effect should be perf, which I haven't measured yet.  Would prefer to do measurements in one thorough pass after the last `except Exception` is removed around cython_agg_general (this plus two PRs) thanks