arrow

ARROW-9291 [R]: Support fixed size binary/list types

some progress here:

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

a <- Array$create(vctrs::list_of(as.raw(1:4), as.raw(1:4)), type = fixed_size_binary(4L))
a
#> Array
#> <fixed_size_binary[4]>
#> [
#>   01020304,
#>   01020304
#> ]
v <- a$as_vector()
v
#> <fixed_size_binary<4>[2]>
#> [[1]]
#> [1] 01 02 03 04
#>
#> [[2]]
#> [1] 01 02 03 04
str(v)
#> fixed_size_binary<4> [1:2]
#> $ : raw [1:4] 01 02 03 04
#> $ : raw [1:4] 01 02 03 04

# we can skip the type= we have an R object that inherits from vctrs_list_of,
# with class arrow_fixed_size_binary and a byte_width attribute
raws <- vctrs::new_list_of(
  list(as.raw(1:4), as.raw(1:4)),
  ptype = raw(),
  byte_width = 4L,
  class = "arrow_fixed_size_binary"
)
a <- Array$create(raws)
a
#> Array
#> <fixed_size_binary[4]>
#> [
#>   01020304,
#>   01020304
#> ]
v <- a$as_vector()
v
#> <fixed_size_binary<4>[2]>
#> [[1]]
#> [1] 01 02 03 04
#>
#> [[2]]
#> [1] 01 02 03 04
str(v)
#> fixed_size_binary<4> [1:2]
#> $ : raw [1:4] 01 02 03 04
#> $ : raw [1:4] 01 02 03 04
```

<sup>Created on 2020-07-07 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9001)</sup>

Closes #7660 from romainfrancois/ARROW-9291/FixedSize

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>