ARROW-6438 [R]: Add bindings for filesystem API

[romainfrancois]

No description provided.

[romainfrancois]

This misses some tests that we can I guess borrow from https://github.com/apache/arrow/blob/ab803e393c9f6873031d0400d1bfe9c039a3b612/python/pyarrow/tests/test_fs.py

[romainfrancois]

@nealrichardson I've added some basic tests. I guess we'd do s3 file system in another PR ?

[nealrichardson]

Yes, we'll do S3 on https://issues.apache.org/jira/browse/ARROW-6439.

[nealrichardson]

Appveyor failed: https://ci.appveyor.com/project/ApacheSoftwareFoundation/arrow/builds/27494817/job/t9yhu00mpe0pcfkp#L4093

> test_check("arrow")
  -- 1. Error: SubTreeFilesystem (@test-filesystem.R#86)  ------------------------
  Unknown error: Underlying filesystem returned path 'C:/Users/appveyor/AppData/Local/Temp/1/RtmpwbPLFA/working_dir/Rtmpu4pBCJ/file9a84a3f668f/DESCRIPTION', which is not a subpath of 'C:/Users/appveyor/AppData/Local/Temp/1\RtmpwbPLFA/working_dir\Rtmpu4pBCJ\file9a84a3f668f/'
  1: st_fs$GetTargetStats(c("DESCRIPTION", "test", "nope", "DESC.txt")) at testthat/test-filesystem.R:86
  2: map(fs___FileSystem__GetTargetStats_Paths(self, x), shared_ptr, class = FileStats)
  3: fs___FileSystem__GetTargetStats_Paths(self, x)
  
  == testthat results  ===========================================================
  [ OK: 992 | SKIPPED: 2 | WARNINGS: 0 | FAILED: 1 ]

[nealrichardson]

Seems straightforward enough for now. I didn't look closely at the cpp code but it doesn't look controversial. Just a couple of notes, plus the Windows build failure.

[nealrichardson]

Looks good except for this documentation warning: https://travis-ci.org/apache/arrow/jobs/586900859#L3378