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ARROW-6622: [R] Normalize paths for filesystem API on Windows

I fixed this in R, though I wonder if that's not enough. At a minimum, the C++ docs should note this requirement (@pitrou maybe you know the best place in the docs to add this?), and I still think it would be nice if all of this path normalization were handled in C++ (cf. https://issues.apache.org/jira/browse/ARROW-6324).

Closes #5445 from nealrichardson/win-fs-fix and squashes the following commits:

515d7105b <Neal Richardson> Rename functions
1775a6d38 <Neal Richardson> Fix two other absolute paths I missed
0dce0d780 <Neal Richardson> Munge paths directly on windows
f03815e07 <Neal Richardson> Normalize paths for filesystem API on Windows

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>