arrow

PARQUET-1701: [C++] Stream API: Add support for optional fields

StreamReader and StreamWriter classes can now support schemata with
optional fields.

Additional input/output operators are available using the
arrow::util::optional class in order to indicate whether optional data
was present when reading, or is present when writing.

Closes #5928 from gawain-bolton/PARQUET-1701_stream_api_optional_fields and squashes the following commits:

e9d80011b <gawain.bolton> Rename tests to conform with codebase.
70d9b3bc6 <gawain.bolton> Improve documentation on input/output operators for required and optional fields.
1efa7ffe6 <gawain.bolton> Updates after code review by emkornfield on 20191229.
de82604ec <gawain.bolton> Eliminate warning on Visual Studio 2017 build
70233da0f <gawain.bolton> PARQUET-1701:  Stream API: Add support for optional fields

Authored-by: gawain.bolton <gawain.bolton@cfm.fr>
Signed-off-by: Wes McKinney <wesm+git@apache.org>