arrow

ARROW-5503 [R]: add read_json()

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

tf <- tempfile()
writeLines('
    { "hello": 3.5, "world": false, "yo": "thing" }
    { "hello": 3.25, "world": null }
    { "hello": 3.125, "world": null, "yo": "\u5fcd" }
    { "hello": 0.0, "world": true, "yo": null }
', tf)

tab <- read_json_arrow(tf)
tab
#> arrow::Table
tab$schema
#> arrow::Schema
#> hello: double
#> world: bool
#> yo: string
as_tibble(tab)
#> # A tibble: 4 x 3
#>   hello world yo
#>   <dbl> <lgl> <chr>
#> 1  3.5  FALSE thing
#> 2  3.25 NA    <NA>
#> 3  3.12 NA    忍
#> 4  0    TRUE  <NA>
```

<sup>Created on 2019-06-11 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9000)</sup>

Author: Romain Francois <romain@rstudio.com>

Closes #4518 from romainfrancois/ARROW-5503/read_json and squashes the following commits:

0314b6a0 <Romain Francois> #include <arrow/...> should be protected by ARROW_R_WITH_ARROW
14cf419e <Romain Francois> + read_json_arrow(as_tibble=)
d469467a <Romain Francois> use ]
57d4d076 <Romain Francois> + read_json_arrow()