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

[andygrove]

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

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

[max-sixty]

There is one of these in errors.rs FYI

[xhochy]

+1, LGTM

[max-sixty]

@andygrove are we still using Schema? The tests now all reference Vecs of fields in a single DataType.

[andygrove]

I think Schema is becoming redundant. I'm still using it in my project to define a RecordBatch but I could switch to using a Vec<Field> instead. Do you want to file a JIRA for removing Schema?

[max-sixty]

OK, though to what extent should we mirror https://arrow.apache.org/docs/metadata.html?

That would make a Schema of Fields.
Struct(Vec<Field>) would go from a variant of DataType to defined as children on a Field, whose type was only Struct?

(tbc these are low confidence and I'm only recently learning, so shoot me down if I'm off)