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

[nealrichardson]

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

[github-actions]

https://issues.apache.org/jira/browse/ARROW-10089

[ianmcook]

Could you add a TODO/comment here describing why this has not yet been changed to a method of the base class ArrowTabular? And another one where $<-.Table is defined

[nealrichardson]

I ended up moving them to ArrowTabular and left stop(TODO) messages in the missing RecordBatch methods.

[ianmcook]

@nealrichardson could you please confirm that this is correct?

• RecordBatch and Table now inherit the synthetic class ArrowTabular
• Array, ChunkedArray, and Scalar now inherit the synthetic class ArrowDatum
• These synthetic classes do not exist in the C++ class hierarchy; they are used here because they simplify S3 method dispatch.

[nealrichardson]

@nealrichardson could you please confirm that this is correct?

• RecordBatch and Table now inherit the synthetic class ArrowTabular
• Array, ChunkedArray, and Scalar now inherit the synthetic class ArrowDatum
• These synthetic classes do not exist in the C++ class hierarchy; they are used here because they simplify S3 method dispatch.

Correct, and I expanded on the comments about that in the code.