Misc performance improvements for nanoplots / vec_*() functions.

[olivroy]

Summary

Improve a little performance for vec_*() by avoiding converting to a tibble and remove some dplyr calls to group_vars().

This could probably be improved further, but saw these quick fixes so I added that.

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

I wonder if this could be improved further. I used the first example of cols_nanoplot()

illness |>
  dplyr::slice_head(n = 10) |>
  gt(rowname_col = "test") |>
  tab_header("Partial summary of daily tests performed on YF patient") |>
  tab_stubhead(label = md("**Test**")) |>
  cols_hide(columns = starts_with("norm")) |>
  fmt_units(columns = units) |>
  cols_nanoplot(
    columns = starts_with("day"),
    new_col_name = "nanoplots",
    new_col_label = md("*Progression*")
  ) |>
  cols_align(align = "center", columns = nanoplots) |>
  cols_merge(columns = c(test, units), pattern = "{1} ({2})") |>
  tab_footnote(
    footnote = "Measurements from Day 3 through to Day 8.",
    locations = cells_column_labels(columns = nanoplots)
  )

Before

This PR:

~8% gain! Not that bad I guess?

[rich-iannone]

LGTM!