refactor: several updates to dsl.rs

[etiennebacher]

No changes visible by user (except for error messages):

• Replace r_result_list by RResult<RPolarsExpr>
• Remove calls to .map() when they are unnecessary
• Update the message when a value is not accepted.

Old:

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

df = pl$DataFrame(
  foo = 1:3,
  bar = c(6, 7, 8),
  ham = c("a", "b", "c")
)

other_df = pl$DataFrame(
  apple = c("x", "y", "z"),
  ham = c("a", "b", "d")
)

df$join(other_df, on = "ham", how = "foobar")
#> Error: Execution halted with the following contexts
#>    0: In R: in $join():
#>    1: The argument [how] caused an error
#>    2: Got value [JoinType choice ('foobar') must be one of 'cross', 'inner', 'left', 'outer', 'semi', 'anti']

New:

Error: Execution halted with the following contexts
   0: In R: in $join():
   1: The argument [how] caused an error
   2: Invalid choice: JoinType ('foobar') must be one of 'cross', 'inner', 'left', 'outer', 'semi', 'anti'

[eitsupi]

Thanks!