Adjust epi_slide_opt output naming; adjust epi_slide* autogrouping; fix tidyselect issue

[brookslogan]

Checklist

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Change explanations for reviewer

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• (Does half of Add .suffix (and .prefix?) arguments to slides #542.)
• Resolves Make epi_slide_opt work with !!varname, maybe !!varnames, maybe !!!varnames #562.
• Resolves Speed up duplicate detection in as_epi_df() #560.
• Resolves Consider whether as_epi_df should ungroup input #573. (epipredict tests pass. epipredict bug identified, worked around, fix filed: cmu-delphi/epipredict#428)
• Resolves Avoid inclusion of epikeys in roll_iqr #575

[brookslogan]

• resolve styler vs linter opinions on line length
• check whether naming overrides forwarded through _mean and _sum
• [-] add .prefix and .suffix naming features to base epi[x]_slide(?) --- but they are not the same feature... .prefix and .suffix for epi[x]_slide would be on the output columns, not on the input columns, and would not need to be glue syntax... might be confusing.
• Update NEWS.md
• [-] vignette updates?
  • Just did minimal changes to keep them running for now. Really should present epi_slide_{sum,mean} first, then the general version.
• auto-grouping&ungrouping by epikey, and ensure epi_slide also auto-ungroups if auto-grouping?
• Update epipredict to not break
  • No update should be needed; epi_slide_opt & co. currently aren't used in epipredict.

[brookslogan]

@nmdefries sorry for all the noise here; I tried to implement just 1 time utility from #494 but forgot it actually relied on the concept of a "unit" time delta/step; fixed that up and added the unit time step helper. Think that's fixed now. Also fixed some CHECK issues.

The core changes, and now these helpers, should hopefully be ready now.

[nmdefries]

Looks good. This is ready to merge aside from failing tests (looks like dplyr isn't loaded in examples).