arrow

ARROW-5625: [R] convert Array of struct type to data frame columns

struct arrays become data frame columns, i.e.

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

tf <- tempfile()
writeLines('
    { "hello": 3.5, "world": false, "yo": "thing", "nuf": {} }
    { "hello": 3.25, "world": null, "nuf": null }
    { "hello": 3.125, "world": null, "yo": "\u5fcd", "nuf": { "ps": 78.0, "house": "Gryffindor"} }
    { "hello": 0.0, "world": true, "yo": null, "nuf": { "ps": 90.0, "house": "Slytherin" } }
  ', tf)

tab1 <- read_json_arrow(tf, as_tibble = FALSE)
array <- tab1$column(3)$data()$chunk(0)
array$field(0)
#> arrow::Array
#> [
#>   null,
#>   null,
#>   78,
#>   90
#> ]
array$as_vector()
#>   ps      house
#> 1 NA       <NA>
#> 2 NA       <NA>
#> 3 78 Gryffindor
#> 4 90  Slytherin
as.data.frame(tab1)
#> # A tibble: 4 x 4
#>   hello world yo    nuf$ps $house
#>   <dbl> <lgl> <chr>  <dbl> <chr>
#> 1  3.5  FALSE thing     NA <NA>
#> 2  3.25 NA    <NA>      NA <NA>
#> 3  3.12 NA    忍        78 Gryffindor
#> 4  0    TRUE  <NA>      90 Slytherin
```

<sup>Created on 2019-06-17 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9000)</sup>

Author: Romain Francois <romain@rstudio.com>

Closes #4593 from romainfrancois/ARROW-5625/struct_arrays and squashes the following commits:

b1f087e9 <Romain Francois> expand on unit test and added comment
17fd51e1 <Romain Francois> effectively use arrays
059c2442 <Romain Francois> lint
394a3d76 <Romain Francois> Converter_Struct::Ingest.*()
a76f2671 <Romain Francois> Converter_Struct::Allocate()
e425c5b2 <Romain Francois> More StrucrtArray methods
bd71f388 <Romain Francois> + methods GetFieldIndex() and GetFieldByName() to arrow::StructType