speed up get_args() helper

[simonpcouch]

With main dev:

library(parsnip)

bench::mark(
  total = fit(linear_reg(), mpg ~ ., mtcars),
  iterations = 25
)
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 total        7.65ms   8.04ms      120.    7.19MB     67.6

With this PR:

#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 total        6.62ms   6.78ms      145.    7.19MB     56.5

Taken together with the other PRs this morning, fit(linear_reg(), mpg ~ ., mtcars) is now free of any calls to select(), filter(), or mutate()!😮

^{Created on 2023-03-17 with reprex v2.0.2}

[EmilHvitfeldt]

nice and clean! approved

[github-actions]

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