Ensure that row ordering doesn't affect summary row calculations

[rich-iannone]

If there was to be row reordering, the intermediate table used to generate summary values wouldn't take that into account, providing erroneous summary calculations. This change ensures that summary row calculations (by group and through the grand summary method) are independent of the final ordering of rows.

Here are some examples that can be used to test the fix.

tbl_1 <-
    tibble::tibble(
      id = c("1", "2", "3", "4", "5", "6"),
      value = c(1, 10, 1, 10, 99, 1),
      group = c("b", "a", "b", "a", "c", "b")
    )

  tbl_2 <-
    tbl_1 %>%
    dplyr::slice(6, 3, 5, 1, 4, 2)

  tbl_3 <-
    tbl_2 %>%
    dplyr::arrange(group, id)

  # Prepare a set gt tables with summary rows (using the same
  # `summary_rows()` call each time)
  gt_tbl_1 <-
    tbl_1 %>%
    dplyr::group_by(group) %>%
    gt(rowname_col = "id") %>%
    summary_rows(groups = TRUE, columns = vars(value), fns = list("sum"))

  gt_tbl_1b <-
    tbl_1 %>%
    gt(rowname_col = "id", groupname_col = "group") %>%
    summary_rows(groups = TRUE, columns = vars(value), fns = list("sum"))

  gt_tbl_2 <-
    tbl_2 %>%
    gt(rowname_col = "id", groupname_col = "group") %>%
    summary_rows(groups = TRUE, columns = vars(value), fns = list("sum"))

  gt_tbl_3 <-
    tbl_3 %>%
    gt(rowname_col = "id", groupname_col = "group") %>%
    summary_rows(groups = TRUE, columns = vars(value), fns = list("sum"))

After the fix is applied, all of the generated tables should have obviously correct summary lines in each of the groups (no matter the arrangement of the table rows). Prior to the fix, the summary rows in each of these gt objects would be wrong (unless it was sorted in just the right way).

Numerous tests were added to verify that different row combinations and methods of specifying groups don't affect the summary value calculations.

Fixes #553.
Fixes #470.

[jcheng5]

Failing case:

tbl_1 <-
  tibble::tibble(
    id = c("1", "2", "3", "4", "5", "6"),
    value = c(1, 10, 1, 10, 99, 1),
    columns = c(2, 20, 2, 20, 198, 2),
    group = c("b", "a", "b", "a", "c", "b")
  )

gt_tbl_1 <-
  tbl_1 %>%
  dplyr::group_by(group) %>%
  gt(rowname_col = "id") %>%
  summary_rows(groups = TRUE, columns = vars(value, columns), fns = list("sum")) %>%
  grand_summary_rows(columns = vars(value, columns), fns = list("sum"))

gt_tbl_1

[jcheng5]

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