Accept lists in select()

[etiennebacher]

Related to #259 (I couldn't deprecate construct_ProtoExprArray though so no problem to close this PR if you want to do it all at once in another PR).

New behaviour:

library(polars)
l_expr = list(
  pl$col("a"),
  pl$col("b")
)

pl$DataFrame(a = 1:3, b = 1:3, c = 1:3)$select(l_expr)
#> shape: (3, 2)
#> ┌─────┬─────┐
#> │ a   ┆ b   │
#> │ --- ┆ --- │
#> │ i32 ┆ i32 │
#> ╞═════╪═════╡
#> │ 1   ┆ 1   │
#> │ 2   ┆ 2   │
#> │ 3   ┆ 3   │
#> └─────┴─────┘
pl$DataFrame(a = 1:3, b = 1:3, c = 1:3)$select(l_expr, "c")
#> shape: (3, 3)
#> ┌─────┬─────┬─────┐
#> │ a   ┆ b   ┆ c   │
#> │ --- ┆ --- ┆ --- │
#> │ i32 ┆ i32 ┆ i32 │
#> ╞═════╪═════╪═════╡
#> │ 1   ┆ 1   ┆ 1   │
#> │ 2   ┆ 2   ┆ 2   │
#> │ 3   ┆ 3   ┆ 3   │
#> └─────┴─────┴─────┘

[sorhawell]

awesome 🏆 🚀

[eitsupi]

I suppose these tests can be simplify by patrick.

[etiennebacher]

I've never used patrick, I find the tests hard to read (but feel free to modify those ofc)

[sorhawell]

I don't mind patrick for large combinatorial tests. Looks neat. I find, that if a patrick test fails, and it is not immediately clear to me why, then I will spend more time than normal to debug the test.