ENH: cythonize groupby.count
closes #7003
- [x] vbench
- ~~axis parameter~~ (this only works in non-cython land)
- [x] tests for `object` dtype if there are none
- [x] datetime64/timedelta64 count

vbench results:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_multi_count                          |   7.3980 | 6814.2579 |   0.0011 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```
