[R] initial R functionality

[asfimport]

Wrapping C++ pointers to arrow objects as R6 classes holding an R external pointer. 

Factory functions for the metadata types, int32(), ...

Factory to create schemas and struct

Create Array, RecordBatch, Table from R vectors and data frames. initially only support integer (int32), numeric (float64) and raw (int8) vectors. 

Reading and Writing record batches and Table to files. 

Reporter: Romain Francois / @romainfrancois
Assignee: Romain Francois / @romainfrancois

PRs and other links:

• GitHub Pull Request #2596

_{Note: This issue was originally created as ARROW-3282. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2596
#2596

[asfimport]

Wes McKinney / @wesm:
We have been leaving completed issues in "Resolved" state. If an issue cannot be resolved for some reason, we transition to "Closed"