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ARROW-15271: [R] Refactor do_exec_plan to return a RecordBatchReader

Ticket title is misleading: this PR actually removes do_exec_plan(). plan$Run() now always returns a RBR; the two cases where Tables are used to post-process ExecPlan results are encapsulated in Run() now.

There is one catch that still needs addressing, but I'll make another jira for it: you can provide schema metadata to the WriteNode but not the other SinkNodes, so anything that preserves R metadata needs to handle that separately because Run() will drop it. ~~This seems to be a limitation of the C++ library.~~ *(edit: I see where I can inject this in `compute::MakeGeneratorReader` in the thing that consumes the sink node, I had made a note about that on the JIRA previously. I still will take this up in ARROW-16607.)*

One other change here: map_batches() now returns a RBR and requires that the function it maps returns something that is coercible to a RecordBatch.

Closes #13170 from nealrichardson/exec-to-rbr

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