TST: platform indexing, xref GH8007
PERF: StataWriter is slow
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.