speed improvement for step_lencode_glm()

[EmilHvitfeldt]

I think i found some evidence that we can improve the speed of step_lencode_glm() significantly

the following shows a rough benchmark. to note

• they produce the same result up to 10^-15
• the ordering of the values are not the same, but doesn't matter as we left_join it on
• this only works for the numeric outcome, but would be easy enough to extend to other supported modes
• old method scales linearly in time with the number of levels of x. new method has same speed

library(embed)
n_obs <- 500000

data <- tibble(
  outcome = rnorm(n_obs),
  x = factor(sample(seq_len(100), n_obs, TRUE))
)

tictoc::tic("old")
res <- recipe(outcome ~ x, data = data) |>
  step_lencode_glm(x, outcome = vars(outcome)) |>
  prep()
tictoc::toc()
#> old: 8.327 sec elapsed


tictoc::tic("new")
tmp <- data |>
  summarise(value = mean(outcome), .by = x)
tictoc::toc()
#> new: 0.007 sec elapsed