arrow

ARROW-6274: [Rust] [DataFusion] Add support for writing results to CSV

I think if we implement a function converting Relation to CSV string, it is a duplicate of some functions in arrow/csv/writer.rs, so I just added a function to export all RecordBatches in Relation.
Also, I encapsulated StringWriter in arrow/utils/string_writer.rs, as a Write trait's implementation, which can be set as arrow::csv::Writer 's parameter.

In fact, it is not an elegant implementation. I have planned to implement a function converting Vec<& RecordBatch> to string, however, the same issue, it is duplicate.

And another way is that we can convert Vec<& RecordBatch> to Vec<Vec<String>> first and convert it into a string or write it into a file, but it may have performance issue when there are many rows.

Any suggestions are welcome. Thanks!

Closes #5577 from hengruo/ARROW-6274 and squashes the following commits:

e89623fc3 <Hengruo Zhang> ARROW-6274: rename 'bow' to 'beginning'
f5a6ae0a0 <Hengruo Zhang> ARROW-6274

Authored-by: Hengruo Zhang <who@hengruo.me>
Signed-off-by: Andy Grove <andygrove73@gmail.com>