arrow

ARROW-8779: [R] Implement conversion to List<Struct>

``` r
library(arrow)
#>
#> Attaching package: 'arrow'
#> The following object is masked from 'package:utils':
#>
#>     timestamp
Array$create(list(data.frame(a = 1:3, b = 11:13)))
#> ListArray
#> <list<item: struct<a: int32, b: int32>>>
#> [
#>   -- is_valid: all not null
#>   -- child 0 type: int32
#>     [
#>       1,
#>       2,
#>       3
#>     ]
#>   -- child 1 type: int32
#>     [
#>       11,
#>       12,
#>       13
#>     ]
#> ]
```

<sup>Created on 2020-06-15 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

However does not yet deal with the original issue:

``` r
library(arrow)
#>
#> Attaching package: 'arrow'
#> The following object is masked from 'package:utils':
#>
#>     timestamp

nrows <- 1:3
df <- tibble::tibble(
    id = 1L,
    data =
      list(
        tibble::tibble(
          a = letters[nrows],
          b = as.integer(nrows),
          c = as.factor(a),
          d = b/2
        )
      )
  )
Table$create(df)
#> Error in Table__from_dots(dots, schema): NotImplemented: Converting vector to arrow type dictionary<values=string, indices=int8, ordered=0> not implemented
```

<sup>Created on 2020-06-15 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

Closes #7435 from romainfrancois/ARROW-8779

Authored-by: Romain Francois <romain@rstudio.com>
Signed-off-by: Wes McKinney <wesm@apache.org>