arrow

ARROW-12693: [R] add unique() methods for ArrowTabular, datasets (#13641)

This implements are pretty thin version of `unique` and some tests:

``` r
library(arrow, warn.conflicts = FALSE)
library(dplyr, warn.conflicts = FALSE)

ir_ds <- arrow_table(iris) %>%
  select(Species)
unique(ir_ds)
#> # A tibble: 3 × 1
#>   Species   
#>   <fct>     
#> 1 setosa    
#> 2 versicolor
#> 3 virginica
```

Authored-by: Sam Albers <sam.albers@gmail.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>