arrow

ARROW-9001: [R] Box outputs as correct type in call_function

call_function() internally unbox to the right R6 class.

This probably needs some more work, e.g. not sure how to deal with this function:

```r
.guess_result_class <- function(arg) {
  # HACK HACK HACK delete this when call_function returns an ArrowObject itself
  if (inherits(arg, "ArrowObject")) {
    return(class(arg)[1])
  } else if (inherits(arg, "array_expression")) {
    return(arg$result_class)
  } else {
    stop("Not implemented")
  }
}
```

Closes #8256 from romainfrancois/ARROW-9001/unbox

Lead-authored-by: Romain Francois <romain@rstudio.com>
Co-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Romain François <romain@rstudio.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>