arrow

ARROW-3816: [R] nrow.RecordBatch method

``` r
library(arrow)
tab <- table(tibble::tibble(x = 1:10, y  = 1:10))
dim(tab)
#> [1] 10  2
nrow(tab)
#> [1] 10
ncol(tab)
#> [1] 2
```

This just implements `dim()`.

`nrow()` and `ncol()` are derived from it:

```r
> nrow
function (x)
dim(x)[1L]
<bytecode: 0x1045a65b8>
<environment: namespace:base>
> ncol
function (x)
dim(x)[2L]
<bytecode: 0x10e2f4dd0>
<environment: namespace:base>
```

Author: Romain Francois <romain@purrple.cat>

Closes #3563 from romainfrancois/ARROW-3816/nrow_RecordBatch and squashes the following commits:

f34efc24 <Romain Francois> export dim.Table
7977678a <Romain Francois> minor name fix
4ebd254a <Romain Francois> dim.(RecordBatch,Table)