perf: revisit epi_slide_opt_archive_one_epikey interface

[brookslogan]

Consider whether epi_slide_opt_archive_one_epikey should:

• Accept a data.table rather than a tibble, and use the fast nesting operation DT[, list(SD = .SD), keyby = <keys>]. (This moves us away from potential generality, though, as tibbles would let us group by packed tibble keys. Make .SD inherit partial key from parent data table in cases where grouping is performed by a subset of the key Rdatatable/data.table#1736 also may impact any changes inside of the function we might make.)
• Not rbind itself, but leave that to the parent function, which may be able to take the resulting lists of tibbles and double-unchop more efficiently (not forming as many medium-size intermediate non-epikey column results). Consider also in parent function whether group_modify should be replaced by group_map and special logic used to prevent forming as many medium-size intermediate epikey column results.