arrow

ARROW-5721: [Rust] Move array related code into a separate module

This moves all array related code into a separate module `array`, and re-export public interfaces.
This allows us to:
1. Split array related implementations into multiple files (e.g., equality, display, etc).
2. Via re-exporting, we can keep the common structs/traits under the `arrow::array` namespace. So,
instead of saying:
```rust
import arrow::builder::Int32Builder;
```
We can say:
```rust
import arrow::array::Int32Builder;
```
which is more accurate.

Author: Chao Sun <sunchao@apache.org>
Author: Chao Sun <sunchao@uber.com>

Closes #4687 from sunchao/ARROW-5721 and squashes the following commits:

9cf953206 <Chao Sun> Fix lint
266d36e6d <Chao Sun> Fix array_data and test warnings
14b8fcb0c <Chao Sun> Rename array_data.rs to data.rs
1083d3660 <Chao Sun>  Move array related code into a separate module