PERF: StataWriter is slow

StataWriter wrote data using scalar operations.  This has been replaced using
numpy's internal binary writer (tofile).

Changes needed to improve performance include:

* Vectorized pandas date to Stata date conversion
* Vectorized null padding
* Conversion to record array

When data contain strings, the old writing paths are still used.
is this path only for timedeltas? is it tested? of u don't have a valid pandas datetime/timedelta you should raise
if it's object dtype then you can I lib.infer_dtypr and act on that (eg say it's date objects or something)

but of its a numpy datetime then it can only be M8[ns] anything else is simply not possible
u can leave the code commented out if u want