Make functions pipe-friendly

[strengejacke]

API-wise I think it would make sense to add a estimate_contrasts() method to apply to estimate_slopes() outputs

estimate_slopes(m) |> estimate_contrasts()

Instead of complexifying the estimate_slopes() itself

Originally posted by @DominiqueMakowski in #301 (comment)

Yes, we would still not recompute from scratch

I reckon a pipe workflow would just be syntactic sugar for:

estimate_contrasts.estimate_means <- function(x) {
  model <- extract_model(x)
  # recompute contrasts
  estimate_contrasts(model, ...)
}