arrow

ARROW-8899 [R] Add R metadata like pandas metadata for round-trip fidelity

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

tab <- Table$create(
  a = structure(1:4, foo = "bar"),
  b = haven::labelled(1:4, label = "description")
)
tab$metadata$r
#> [1] "A\n3\n262144\n197888\n5\nUTF-8\n19\n2\n254\n531\n2\n1026\n1\n262153\n3\nfoo\n16\n1\n262153\n3\nbar\n254\n1026\n1\n262153\n5\nlabel\n16\n1\n262153\n11\ndescription\n1026\n1\n262153\n5\nclass\n16\n3\n262153\n14\nhaven_labelled\n262153\n10\nvctrs_vctr\n262153\n7\ninteger\n254\n1026\n1\n262153\n5\nnames\n16\n2\n262153\n1\na\n262153\n1\nb\n254\n"
arrow:::.arrow_unserialize(tab$metadata$r)
#> [[1]]
#> NULL
#>
#> [[2]]
#> [[2]]$a
#> [[2]]$a$foo
#> [1] "bar"
#>
#>
#> [[2]]$b
#> [[2]]$b$label
#> [1] "description"
#>
#> [[2]]$b$class
#> [1] "haven_labelled" "vctrs_vctr"     "integer"

df <- as.data.frame(tab)
df$a
#> [1] 1 2 3 4
#> attr(,"foo")
#> [1] "bar"
df$b
#> <labelled<integer>[4]>: description
#> [1] 1 2 3 4
```

<sup>Created on 2020-06-23 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9001)</sup>

`metadata$r$` gets a serialized string of a list with 2 things:
  - attributes of the data frame itself
  - named list of attributes of each column (possibly NULL)

Closes #7524 from romainfrancois/ARROW-8899/metadata

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