ARROW-6539: [R] Provide mechanism to write out old format

[romainfrancois]

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3c13d22). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #5420   +/-   ##
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  Coverage          ?   76.11%           
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  Files             ?       56           
  Lines             ?     3576           
  Branches          ?        0           
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  Hits              ?     2722           
  Misses            ?      854           
  Partials          ?        0

Impacted Files	Coverage Δ
r/src/arrow_types.h	96% <ø> (ø)
r/R/record-batch-writer.R	60% <ø> (ø)
r/src/parquet.cpp	75% <ø> (ø)
r/R/arrowExports.R	71.96% <100%> (ø)
r/src/recordbatchwriter.cpp	100% <100%> (ø)
r/src/arrowExports.cpp	71.66% <100%> (ø)

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[romainfrancois]

@javierluraschi is this enough for sparklyr needs ?

[nealrichardson]

sparklyr uses write_arrow: https://github.com/rstudio/sparklyr/blob/master/R/core_arrow.R#L19

And when writing to a raw vector, it uses RecordBatchStreamWriter: https://github.com/apache/arrow/blob/master/r/R/write-arrow.R#L86

So I think we're covered. I'll merge.