REF: use standard pattern in normalize_i8_timestamps
From here the plan is to move to passing `int64_t*` which will allow us to share code between the scalar/vector versions of this function
This one seems to have caused a significant performance regression: https://pandas.pydata.org/speed/pandas/#timeseries.DatetimeAccessor.time_dt_accessor_normalize?p-t='UTC'&commits=4fcc6a66

Is that expected / accepted? yah, i was surprised to see that the cython version is non-trivially slower than the numpy version.  i have one long-shot guess at a way to speed up the cython version, will try that out today. 