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.
ok, I thought you are determining the fixed string dtype from here? if its older code, can you remove / comment out?