arrow

ARROW-14694: [R] Let me dput a schema

This adds `$code()` to `DataType` objects in R, which is then used by `Schema$code()`.

In both case, `$code()` produces an R call, e.g.

``` r
library(arrow, warn.conflicts = FALSE)
#> See arrow_info() for available features

int32()$code()
#> int32()
struct(a = timestamp())$code()
#> struct(a = timestamp(unit = "s"))

schema(
  a = float64(),
  b = list_of(large_utf8()),
  c = struct(
    d = fixed_size_binary(byte_width = 7L),
    e = null()
  )
)$code()
#> schema(a = float64(), b = list_of(large_utf8()), c = struct(d = fixed_size_binary(byte_width = 7L),
#>     e = null()))
```

<sup>Created on 2021-11-22 by the [reprex package](https://reprex.tidyverse.org) (v2.0.1.9000)</sup>

Closes #11751 from romainfrancois/ARROW_14694_dput_Schema

Lead-authored-by: Romain Francois <romain@rstudio.com>
Co-authored-by: Romain François <romain@rstudio.com>
Co-authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>