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ARROW-8867: [R] Support converting POSIXlt type

Also contains some test refactor, documentation, and slight tweaks as followup to ARROW-8899.

Closes #7601 from nealrichardson/posixlt

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