ARROW-14989: [R] Update num_rows methods to output doubles not integers to prevent integer overflow

[thisisnic]

This PR enables num_rows() methods to be called on Table and RecordBatch objects without integer overflow when the value of num_rows() is higher than .Machine$integer.max. I originally wrote some tests but they take ages to run and crashed on CI anyway so I removed them, but they can be seen in https://github.com/apache/arrow/pull/13482/commits/e7cf8a66beab6d1b7d85304362086b6205a31279/.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-14989

[github-actions]

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[paleolimbot]

These changes should get it to work...the explicit keyword disables C++'s automatic generation of implicit construction via assignment (i.e., r_vec_size some_variable = (R_x_len_t) 123;), which is what happens when you return something (maybe I'm getting the details slightly wrong here but that's how I've internalized it...the documentation on this is not awesome: https://en.cppreference.com/w/cpp/language/explicit ).

It would be a little more slick to define the proper converting constructor so that you can just return some_int64; but I don't know the exact incantation to make that work.

[ursabot]

Benchmark runs are scheduled for baseline = 41f8bdf and contender = 7124baf. 7124baf is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.0% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.27% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.15% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 7124bafb ec2-t3-xlarge-us-east-2
[Finished] 7124bafb test-mac-arm
[Finished] 7124bafb ursa-i9-9960x
[Finished] 7124bafb ursa-thinkcentre-m75q
[Finished] 41f8bdfe ec2-t3-xlarge-us-east-2
[Failed] 41f8bdfe test-mac-arm
[Finished] 41f8bdfe ursa-i9-9960x
[Finished] 41f8bdfe ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[nealrichardson]

I think there's at least one more row count that needs this attention: https://github.com/apache/arrow/blob/master/r/src/dataset.cpp#L514