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ARROW-3762: [C++/Python] Support reading Parquet BYTE_ARRAY columns containing over 2GB of data

This patch ended up being a bit more of a bloodbath than I planned: please accept my apologies.

Associated changes in this patch:

* Split up builder.h/builder.cc into a new arrow/array directory. Public arrow/builder.h API preserved. I think this code is going to keep growing more specialized components, so I think we should get out ahead of it by having a subdirectory to contain files related to implementation details
* Implement ChunkedBinaryBuilder, ChunkedStringBuilder classes, add tests and benchmarks
* Deprecate parquet::arrow methods returning Array
* Allow implicit construction of Datum from its variant types (makes for a lot nicer syntax)

As far as what code to review, focus efforts on

* src/parquet/arrow
* src/arrow/array/builder_binary.h/cc, array-binary-test.cc, builder-benchmark
* src/arrow/compute changes
* Python changes

I'm going to tackle ARROW-2970 which should not be complicated after this patch; I will submit that as a PR after this is reviews and merged.

Author: Wes McKinney <wesm+git@apache.org>

Closes #3171 from wesm/ARROW-3762 and squashes the following commits:

822451280 <Wes McKinney> Fix int conversion warning on Windows
695ffc9df <Wes McKinney> Remove unimplemented and unused ChunkedBinaryBuilder ctor
5a525115c <Wes McKinney> Use strnlen to compute string length. Inline BinaryBuilder::AppendNextOffset
b90eb4b71 <Wes McKinney> Restore sstream include to pretty_print.cc
3669201be <Wes McKinney> Fix deprecated API use
5fdbbb261 <Wes McKinney> Rename columnar/ directory to array/
8ffaec1ef <Wes McKinney> Address preliminary code comments. Check in missing files
81e787c69 <Wes McKinney> Fix up Python bindings, unit test
2efae064c <Wes McKinney> Finish scaffolding. Get fully compiling again and original parquet-arrow test suite passing
3d075e4aa <Wes McKinney> Additional refactoring to make things chunked. Allow implicit construction of arrow::compute::Datum
922811278 <Wes McKinney> More refactoring
716322377 <Wes McKinney> Split up builder.h, builder.cc into smaller headers, compilation units. add failing test case for ARROW-3762. Add ChunkedBinaryBuilder, make BinaryBuilder Append methods inline