arrow

ARROW-11699: [R] Implement dplyr::across() for mutate()

This PR introduces a partial implementation for `dplyr::across()` when called within `dplyr::mutate()`.

``` r
arrow_table(iris) %>%
  mutate(across(starts_with("Sepal"), list(round, sqrt)))
#> Table (query)
#> Sepal.Length: double
#> Sepal.Width: double
#> Petal.Length: double
#> Petal.Width: double
#> Species: dictionary<values=string, indices=int8>
#> Sepal.Length_1: double (round(Sepal.Length, {ndigits=0, round_mode=HALF_TO_EVEN}))
#> Sepal.Length_2: double (sqrt_checked(Sepal.Length))
#> Sepal.Width_1: double (round(Sepal.Width, {ndigits=0, round_mode=HALF_TO_EVEN}))
#> Sepal.Width_2: double (sqrt_checked(Sepal.Width))
#>
#> See $.data for the source Arrow object
```

I've opened a number of follow-up tickets for the tasks needed to be done to provide a more complete implementation:
* [ARROW-17362: [R] Implement dplyr::across() inside summarise()](https://issues.apache.org/jira/browse/ARROW-17362)
* [ARROW-17387: [R] Implement dplyr::across() inside filter()](https://issues.apache.org/jira/browse/ARROW-17387)
* ~[ARROW-17364: [R] Implement .names argument inside across()](https://issues.apache.org/jira/browse/ARROW-17364)~ (now done in this PR, will close it once this is merged)
* [ARROW-17366: [R] Support purrr-style lambda functions in .fns argument to across()](https://issues.apache.org/jira/browse/ARROW-17366)

Closes #13786 from thisisnic/ARROW-11699_across

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>