arrow

ARROW-11832: [R] Handle conversion of extra nested struct column

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

df <- structure(list(col = structure(list(structure(list(list(structure(1))), class = "inner")), class = "outer")), class = "data.frame", row.names = c(NA, -1L))
Array$create(df)
#> StructArray
#> <struct<col: list<item: list<item: list<item: double>>>>>
#> -- is_valid: all not null
#> -- child 0 type: list<item: list<item: list<item: double>>>
#>   [
#>     [
#>       [
#>         [
#>           1
#>         ]
#>       ]
#>     ]
#>   ]
```

<sup>Created on 2021-04-07 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

Closes #9919 from romainfrancois/ARROW_11832/vec_size

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