arrow

ARROW-3855: [Rust] Schema/Field/Datatype now have derived serde traits

This PR makes `Schema`, `Field`, `DataType` serializable using the serde crate. This approach supports serialization to numerous binary and text formats supported by the serde crate.

The main benefit is to allow users of the Arrow crate to serialize structs that reference Arrow types (for example, allowing a logical query plan to be serialized and sent over the network).

Note that this does not change the custom JSON serialization that is already in place for serializing in the specific format specified in `format/Metadata.md`.

Author: Andy Grove <andygrove73@gmail.com>

Closes #3016 from andygrove/ARROW-3855 and squashes the following commits:

329da92 <Andy Grove> Merge branch 'master' into ARROW-3855
d988cc6 <Andy Grove> cargo fmt
bd8375d <Andy Grove> Schema/Field/Datatype now have derived serde traits