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ARROW-60: [C++] Struct type builder API

Implement the basic classes, `StructArray` and `StructBuilder,` meanwhile,

add the perspective test cases for them.

Other necessary methods will be added subsequently.

Author: fengguangyuan <root@node20.(none)>

Closes #66 from fengguangyuan/ARROW-60 and squashes the following commits:

190967f [fengguangyuan] ARROW-60: [C++] Struct type builder API Add field index and TODO comment.
ae74c80 [fengguangyuan] ARROW-60: Struct type builder API Add RangeEquals method to implement Equals method.
fa856fd [fengguangyuan] ARROW-60:[C++] Struct typebuilder API Modify Validate() refered to the specification.
bfabdc1 [fengguangyuan] ARROW-60: Struct type builder API Refine the previous committed patch. Add validate methods for testing StructArray and StructBuilder. TODO, Equals methods also need to be tested, but now it's not convient to do it.
5733de7 [fengguangyuan] ARROW-60: Struct type builder API