[R] Make decimal128 data available in R without requiring lossy conversions

[asfimport]

Reporter: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #2845

_{Note: This issue was originally created as ARROW-3628. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
@romainfrancois Do you still plan to work on this?

cc @nealrichardson

[asfimport]

Neal Richardson / @nealrichardson:
There's still no (to my knowledge) real way to represent and manipulate Decimal128 data in R. We have the lossy conversion (to float64) implemented from Arrow to R. If you want lossless handling of Decimal* data, there's now the option to do all of the manipulation in Arrow with its richer type system, driving it from R.

I'll close this but we can revisit if/whenever a native R implementation for Decimal data appears.