[R] Implement RPrimitiveConverter for Decimal type

[asfimport]

This succeeds:

Array$create(1)$cast(decimal(10, 2))

but this fails:

Array$create(1, type = decimal(10, 2))

with error:

NotImplemented: Extend

because the Extend method of the RPromitiveConverter class for the Decimal type is not yet implemented.

The error is thrown here: 

arrow/r/src/r_to_arrow.cpp

Line 601 in 7184c3f

return Status::NotImplemented("Extend");

Reporter: Ian Cook / @ianmcook
Assignee: Dewey Dunnington / @paleolimbot

Related issues:

• [C++] Error when creating Table and using schema with decimal type (is duplicated by)
• [R] Creating Decimal type using the example fails (relates to)
• [R] Unit tests for decimals / ARROW-4565 (relates to)
• [R] Use Converter API to convert SEXP to Array/ChunkedArray (relates to)

PRs and other links:

• GitHub Pull Request #15211

_{Note: This issue was originally created as ARROW-11631. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
@romainfrancois  do you know if this would be straightforward to implement with the Converter API? If you could provide some pointers on how to implement this (or let us know what the obstacles are if it's not so simple) that would be wonderful!

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[asfimport]

Ian Cook / @ianmcook:
@thisisnic @paleolimbot  do you think it would be possible to get this done for 11.0.0? In the past I think Romain was the only person who really understood this part of the code, but I think Dewey and others might understand it better now?

[asfimport]

Dewey Dunnington / @paleolimbot:
I'm happy to give it a go...not having this implemented is a pain and I've run into it multiple times now. Thanks for bringing it up!