arrow

ARROW-5718: [R] auto splice data frames in record_batch() and table()

elements of `...` that are unnamed in `record_batch(...)` or `table(...)` are automatically spliced:

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

df <- tibble(x = 1:10, y = letters[1:10])
```

``` r

# auto splicing
batch <- record_batch(df, z = 1:10)
as_tibble(batch)
#> # A tibble: 10 x 3
#>        x y         z
#>    <int> <chr> <int>
#>  1     1 a         1
#>  2     2 b         2
#>  3     3 c         3
#>  4     4 d         4
#>  5     5 e         5
#>  6     6 f         6
#>  7     7 g         7
#>  8     8 h         8
#>  9     9 i         9
#> 10    10 j        10

# same as explicit splicing
batch <- record_batch(!!!df, z = 1:10)
as_tibble(batch)
#> # A tibble: 10 x 3
#>        x y         z
#>    <int> <chr> <int>
#>  1     1 a         1
#>  2     2 b         2
#>  3     3 c         3
#>  4     4 d         4
#>  5     5 e         5
#>  6     6 f         6
#>  7     7 g         7
#>  8     8 h         8
#>  9     9 i         9
#> 10    10 j        10
```

``` r

# auto splicing
tab <- table(df, z = 1:10)
as_tibble(tab)
#> # A tibble: 10 x 3
#>        x y         z
#>    <int> <chr> <int>
#>  1     1 a         1
#>  2     2 b         2
#>  3     3 c         3
#>  4     4 d         4
#>  5     5 e         5
#>  6     6 f         6
#>  7     7 g         7
#>  8     8 h         8
#>  9     9 i         9
#> 10    10 j        10

# same as explicit splicing
tab <- table(!!!df, z = 1:10)
as_tibble(tab)
#> # A tibble: 10 x 3
#>        x y         z
#>    <int> <chr> <int>
#>  1     1 a         1
#>  2     2 b         2
#>  3     3 c         3
#>  4     4 d         4
#>  5     5 e         5
#>  6     6 f         6
#>  7     7 g         7
#>  8     8 h         8
#>  9     9 i         9
#> 10    10 j        10
```

In particular, this gives us back `record_batch(<data.frame>)` and `table(<data.frame>)` :

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

df <- tibble(x = 1:10, y = letters[1:10])
record_batch(df)
#> arrow::RecordBatch
table(df)
#> arrow::Table
```

Author: Romain Francois <romain@rstudio.com>

Closes #4704 from romainfrancois/ARROW-5718/df_auto_splice and squashes the following commits:

79a70412c <Romain Francois> more tests about auto splicingh in record_batch()
28bc470c9 <Romain Francois> + comment
61902ab7c <Romain Francois> table() auto splicing
0a4892c21 <Romain Francois> record_batch() auto splicing of data frames
5eabd202a <Romain Francois> trim redundant code