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 of string columns to fixed width numpy string arrays
* Conversion to record array
Merge pull request #8079 from bashtage/stata-write-performance

PERF: StataWriter is slow