ENH: Allow numba aggregations to return non-float64 results

For example, `groupby.sum` can return an `int64` output given an `int64` input

```
In [2]: pd.DataFrame(range(2)).groupby(level=0).sum()
Out[2]:
   0
0  0
1  1
```

To enable this for our numba aggregations, I think we would need to specify specific signatures.