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ARROW-4506: [Ruby] Add Arrow::RecordBatch#raw_records

I want to add Arrow::RecordBatch#raw_records method to convert a record batch object to a nested array.

This is the first step to implement the feature.  The following things are out of scope of this pull-request.

- Conversion of half-float values to Ruby's float.
- Unit treatment of Time32 and Time64
- Conversion of the following compound data types to Ruby objects:
  - ListType
  - StructType
  - UnionType
  - DictionaryType

### TODO:

- [x] Extracting raw values of HalfFloatArray
- [x] Extracting ListArray
- [x] Extracting StructArray
- [x] Extracting SparseUnionArray
- [x] Extracting DenseUnionArray
    - [x] FixedSizeBinary
    - [x] Date32
    - [x] Date64
    - [x] Timestamp
    - [x] Decimal128
    - [x] Struct
    - [x] Dictionary
- [x] Extracting indices of DictionaryArray
- [x] Make CI passed
- [x] Add benchmark script

Author: Kenta Murata <mrkn@mrkn.jp>
Author: Kouhei Sutou <kou@clear-code.com>

Closes #3587 from mrkn/raw_records and squashes the following commits:

00197e4b <Kouhei Sutou> Split test files
0d0d5170 <Kouhei Sutou> Replace a large StructArray test with small tests
61f9774b <Kouhei Sutou> Use {"field_name" => value} for union value
5fa9ba03 <Kouhei Sutou> Finish replacing tests for StructArray
83bf55a3 <Kouhei Sutou> Add support for nested StructArray
c065a8a2 <Kouhei Sutou> Add tests for StructArray#raw_records
795c96a8 <Kouhei Sutou> Add support for "_" for data type name
87ab55da <Kouhei Sutou> Add support for nil value as NULL for struct field value
08b85c81 <Kouhei Sutou> Add support for nested list
4d78da49 <Kouhei Sutou> Remove resolved TODO
024506f1 <Kouhei Sutou> Remove needless tests
e30a9e73 <Kouhei Sutou> Add support for nil in ListArrayBuilder#append_values
55066950 <Kouhei Sutou> Add primitive array tests
67808a2b <Kouhei Sutou> Add support for building BinaryArray
44779252 <Kouhei Sutou> Add tests for primitive arrays
cb3cb471 <Kouhei Sutou> Add support for NullArray
728b82c9 <Kouhei Sutou> Reduce scope
021dbee2 <Kouhei Sutou> Use .cpp for C++
4d01f85b <Kouhei Sutou> Use constexpr
7f5d6cc3 <Kouhei Sutou> Fix style
db1e1c25 <Kouhei Sutou> Remove needless reference
028e3c4a <Kouhei Sutou> Use auto
ce67b5bb <Kouhei Sutou> Stop reusing block argument name
750afff5 <Kouhei Sutou> Use .cpp for C++ extension
a2032754 <Kouhei Sutou> Fold a long line
46d63aeb <Kouhei Sutou> Use auto
03233bd5 <Kouhei Sutou> Use Red Arrow in build directly
3a776306 <Kouhei Sutou> Fix package name for MSYS2
022dd073 <Kenta Murata> Fix the benchmark for dictionary array
3e18e85f <Kenta Murata> Rename a benchmark file
2c6142c8 <Kenta Murata> Rename a directory
40595617 <Kenta Murata> Fix benchmark task
5d380fdc <Kenta Murata> Use values between 2**16 and 2**32-1 for testing UInt32Array
d0b8d0fc <Kenta Murata> Fix styling
f41550d0 <Kenta Murata> Stop using precomputed scales of time unit
bdf7090e <Kenta Murata> Remove needless scope blocks
8e81e9b2 <Kouhei Sutou>  Implement converters based on visitor
31ca243a <Kenta Murata> Add extension files in the gem package
ec11062b <Kenta Murata> Add arrow ext dir in $LOAD_PATH
d0a4733b <Kenta Murata> Fix benchmark against the removal of convert_decimal: option
24d14800 <Kenta Murata> Guard RVAL2GOBJ by rb::protect
73063288 <Kenta Murata> Drop convert_decimal: option
ba96767a <Kenta Murata> Remove a needless member function
dfa6b4ae <Kenta Murata> Introduce require_extension_library method to load arrow.so
ee8dcccf <Kenta Murata> Avoid using rb_str_new_cstr
7e51f7a8 <Kenta Murata> Fix variable names in test
8d1c36e4 <Kenta Murata> Use #pragma once
bba7eb4e <Kenta Murata> Rename a variable
b47fbfeb <Kenta Murata> Use GOBJ2RVAL_UNREF correctly
4a73d5eb <Kenta Murata> Use auto instead of VALUE
d59d72be <Kenta Murata> Put some codes out side of rb::protect block
9995dec9 <Kenta Murata> Use rb_enc_str_new with rb_ascii8bit_encoding for binary string creation
e0b26563 <Kenta Murata> Replace assert with DCHECK
70ccc327 <Kenta Murata> Make cArrowRecordBatch a local variable
16cbe8d9 <Kenta Murata> Use rb::RawMethod
d6b7d3da <Kenta Murata> Remove needless require
ed507393 <Kenta Murata> Fix styling
da2c49f6 <Kenta Murata> Rename rb_cDate to cDate
4645c17c <Kenta Murata> Rename cRecordBatch to cArrowRecordBatch
2c410cf6 <Kenta Murata> Remove namespace comments
55df564d <Kenta Murata> Rename files
0a4016b3 <Kenta Murata> Replace license headers
180c7c3e <Kenta Murata> Use static timestamp_range in benchmark
1413aa6b <Kouhei Sutou> Set PKG_CONFIG_PATH to build Red Arrow
509af8ff <Kenta Murata> Add benchmark task
e3fa4e62 <Kenta Murata> Fix word usage
31c8a270 <Kenta Murata> Use double quotations
9568a371 <Kenta Murata> Remove redundant sub test cases
7fb64a9b <Kenta Murata> Remove parentheses with empty argument
5bdd9952 <Kenta Murata> Remove needless require
56634502 <Kenta Murata> Remove arrow_ruby_compile function
794c7a27 <Kenta Murata> Revert needless changes
be026ed1 <Kouhei Sutou> Fix style
c0b7442d <Kouhei Sutou> Add "compile" task
99442745 <Kouhei Sutou> Add support "rake clean" and "rake clobber"
56bbc15b <Kouhei Sutou> Run extconf.rb automatically in test/run-test.rb
4d9440d0 <Kouhei Sutou> Use Ext++
d16b6e1d <Kouhei Sutou> Sort alphabetically
6217e862 <Kouhei Sutou> Add support for auto package install
08818907 <Kouhei Sutou> Remove rake-compiler dependency
38d6f530 <Kenta Murata> Make the default value of conver_decimal true
f3cd6724 <Kenta Murata> Move gem entries from Gemfile into gemspec
2b97d3e3 <Kenta Murata> Fix benchmarks
0a9f9ece <Kenta Murata> Fix the random state of Faker in benchmark
26c7ea17 <Kenta Murata> Add benchmark scripts
7eaa0690 <Kenta Murata> Separate raw_records test
de943457 <Kenta Murata> Fix missing const modifiers
40c4966c <Kenta Murata> Support Struct in Union
cba224a3 <Kenta Murata> Support Dictionary in Union
95517e3a <Kenta Murata> Add tests for dense union in dense union
86f8bc11 <Kenta Murata> Fix travis script
82ce1bcb <Kenta Murata> Add license comment
9df5765a <Kenta Murata> Refactoring test
5016c6a9 <Kenta Murata> Support Dictionary in UnionArray
462480d6 <Kenta Murata> Support Date32, Date64, and Timestamp in UnionArray
0d358a3c <Kenta Murata> Refactoring
5a2276bf <Kenta Murata> Use non-default field name for a list in a record batch
6b9de32d <Kenta Murata> Add support of FixedSizeBinary in Union
786e4388 <Kenta Murata> Refactoring of Decimal128 converter
fcd3ab95 <Kenta Murata> Support SparseUnion
ce10bcbd <Kenta Murata> Support Decimal128 in DenseUnion
5b14d106 <Kenta Murata> Add partial support of DenseUnion
432f05ae <Kenta Murata> Fix encoding bug
94fe1c48 <Kenta Murata> Add tentative support of HalfFloat
72a64f5e <Kenta Murata> Refactoring
61f0bc50 <Kenta Murata> Support Dictionary indices
63876209 <Kenta Murata> Support Struct
223db821 <Kenta Murata> Use RETURN_NOT_OK
cead59bb <Kenta Murata> Save errinfo if rb::error created from state
0186066c <Kenta Murata> Extract ArrayConverter class
294c7496 <Kenta Murata> Supply PKG_CONFIG_PATH to rake compile
53fa1ffd <Kenta Murata> Fix CI script
17a69c2a <Kenta Murata> Support List
88119d5a <Kenta Murata> Remove pure-Ruby version
aa52a174 <Kenta Murata> Tweak comment and error message
6c8a2e2c <Kenta Murata> Add tentative supports of Time32 and Time64
d33d77d2 <Kenta Murata> Support Timestamp
72935bc9 <Kenta Murata> Support Date32 and Date64
93518946 <Kenta Murata> Use rb_jump_tag to raise deferred exception
476f544b <Kenta Murata> Add convert_decimal kwarg
54e59e78 <Kenta Murata> Fix VisitValue for nil
5f7a78ca <Kenta Murata> Use RawRecordsBuilder
d8d54d67 <Kenta Murata> Add RawRecordsBuilder
4d6d6392 <Kenta Murata> Update test case
8d68d5bc <Kenta Murata> Add a partial native implementation of RecordBatch#raw_records
25a1925c <Kenta Murata> Add test and tentative implementation of RecordBatch#raw_records