PERF: StataReader is slow due to excessive lookups for missing vales

Previous versions of StataReader did not correctly check for missing values.
This was fixed in a previous PR, but these checks had previously been
implemented on a value-by-value basis.  This has now been changed to a
vectorized version that is orders of magnitude faster.

Adds support for preserving Stata missing values when importing data using
the convert_missing argument.  When True, missing values will be returned as
StataMissingValue objects.  Test cases for missing value conversion have been
included.

Additionally, a benchmark was added to monitor performance issues in the
future.

Closes #8040
Merge pull request #8045 from bashtage/stata_unpack_performance

PERF: StataReader is slow due to excessive lookups for missing vales