move src/datetime.pxd funcs to np_datetime and fix misleading names
The name `_pydatetime_to_dts` is misleading, since `dts` is the variable names used for `pandas_datetimestruct` objects, while the func actually returns an int64.  This renames it to `_pydatetime_to_dt64`.

The analogous function for `datetime.date` has the non-misleading name `_date_to_datetime64`.  For consistency this is renamed to `_pydate_to_dt64`.

Both are moved to `np_datetime`, and have their inputs typed as `datetime` and `date` instead of `object`.
