Enable $explode() for Data/LazyFrame

[etiennebacher]

To merge after #308. I just followed the code for sort_by()

library(polars)
df = pl$DataFrame(
  letters = c("a", "a", "b", "c"),
  numbers = list(1, NULL, c(4, 5), c(6, 7, 8)),
  numbers2 = list(1, NULL, c(4, 5), c(6, 7, 8))
)
df$explode("numbers", pl$col("numbers2"))
#> shape: (7, 3)
#> ┌─────────┬─────────┬──────────┐
#> │ letters ┆ numbers ┆ numbers2 │
#> │ ---     ┆ ---     ┆ ---      │
#> │ str     ┆ f64     ┆ f64      │
#> ╞═════════╪═════════╪══════════╡
#> │ a       ┆ 1.0     ┆ 1.0      │
#> │ a       ┆ null    ┆ null     │
#> │ b       ┆ 4.0     ┆ 4.0      │
#> │ b       ┆ 5.0     ┆ 5.0      │
#> │ c       ┆ 6.0     ┆ 6.0      │
#> │ c       ┆ 7.0     ┆ 7.0      │
#> │ c       ┆ 8.0     ┆ 8.0      │
#> └─────────┴─────────┴──────────┘

[sorhawell]

@etiennebacher I have improved robj_to! a bit . Ideally use of lapply(...,wrap_e_result) and ProtoExprArr can now be replaced with robj_to!(Vec, Expr) or robj_to!(Vec, ExprCol)[str_as_lit=FALSE].

robj_to! makes some very eleborate error messages with class

> err = pl$DataFrame(list(a = list(1:3,11:12)))$explode(list("a",complex())) |>
+   result() |>
+   unwrap_err()
> 
> #get contexts, call and info
> err$contexts()
$BadArgument
[1] "columns"

$Hint
[1] "element no. [2] "

$PlainErrorMessage
[1] "cannot be converted into an Expr"

$BadValue
[1] "Rvalue: [], Rsexp: Complexes, Rclass: [\"complex\"]"

$PlainErrorMessage
[1] "cannot make a column expression from:  cplx(0) "

> err$get_rcall()
[1] "unwrap_err(result(pl$DataFrame(list(a = list(1:3, 11:12)))$explode(list(\"a\", \n    complex()))))"
> err$get_rinfo()
[1] "in explode():"
> 

[sorhawell]

showcase new catch any R error

> showcase = function() {
   stop("imma simple R err") |> 
     result() |>  #catch result and upgrade any raised error to RPolarsErr
     map_err(\(err) err$
               plain("happend while showcasing RPolarsErr")$
               when("talking about RPolarsErr")
     ) |>
     unwrap(" in showcase:")
}
> showcase()
Error: Execution halted with the following contexts
0: In R: in showcase:
0: During function call [showcase()]
1: When talking about RPolarsErr
2: happend while showcasing RPolarsErr
3: imma simple R err

[etiennebacher]

New error handling looks really nice!

[sorhawell]

thanks @etiennebacher ! Looks good.

Requested to merge after 308. Merging whenever also ok I think.