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ARROW-10089: [R] inject base class for Array, ChunkedArray and Scalar

The impetus for this change was to get S3 Ops method dispatch to work for Array == Scalar. This is tested in the third commit.

Closes #9345 from nealrichardson/arrow-datum

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