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ARROW-2385: [Rust] implement to_json for DataType and Field

Note that this PR also moves some tests for comparing arrays from Array to Buffer<T> and removes some redundant code that was implemented before it was possible to get a type-safe Iterator from Buffer<T>.

This change was made in this PR because the serde_json crate's macros pretty much forced me to address this now.

Author: Andy Grove <andygrove73@gmail.com>

Closes #1829 from andygrove/schema_json and squashes the following commits:

6b5281f <Andy Grove> fix issues that stopped code compiling with Rust 1.25.0
6af8963 <Andy Grove> rustfmt
ce2e56d <Andy Grove> remove commented out code
0ba3a77 <Andy Grove> can parse types and fields from json
c9ace3f <Andy Grove> implement to_json for DataType and Field