Make it easier to create a DataFrame with a single list-column

[etiennebacher]

To create a DataFrame with a single list column, we need to wrap the input in list() which is annoying:

library(polars)
options(polars.do_not_repeat_call = TRUE)

pl$DataFrame(list(x = list(1:2, 3:10)))
#> shape: (2, 1)
#> ┌──────────────┐
#> │ x            │
#> │ ---          │
#> │ list[i32]    │
#> ╞══════════════╡
#> │ [1, 2]       │
#> │ [3, 4, … 10] │
#> └──────────────┘

# this should work
pl$DataFrame(x = list(1:2, 3:10))
#> Error: Execution halted with the following contexts
#>    0: In R: in $DataFrame():
#>    1: Encountered the following error in Rust-Polars:
#>          Series length 2 doesn't match the DataFrame height of 8

Note that there's no problem to create a DataFrame with >= 2 list-columns:

pl$DataFrame(x = list(1:2, 3:10), y = list(1:2, 3:10))
#> shape: (2, 2)
#> ┌──────────────┬──────────────┐
#> │ x            ┆ y            │
#> │ ---          ┆ ---          │
#> │ list[i32]    ┆ list[i32]    │
#> ╞══════════════╪══════════════╡
#> │ [1, 2]       ┆ [1, 2]       │
#> │ [3, 4, … 10] ┆ [3, 4, … 10] │
#> └──────────────┴──────────────┘