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ARROW-7018: [R] Non-UTF-8 data in Arrow <--> R conversion

Sprinkles `Rf_translateCharUTF8` a few places. I tried to add tests for all of the different scenarios I could think of where we could have non-UTF strings.

Also includes `$` and `[[` methods for `Schema` objects.

Closes #7527 from nealrichardson/r-utf8

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