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
TST: TestYahooOptions issue
ENH: add support dtype='category' in Series constructor
BUG: fix reindexing to an all-nan Categorical (GH8076)
Merge pull request #8075 from jreback/cat2

ENH: add support dtype='category' in Series constructor
BUG/ENH/API: ewm* debiasing, 'adjust', min_periods; rolling/expanding_var/std for single value