Consider reorganization of `epi_slide*` functions

[brookslogan]

Consider

• Renaming epi_slide_opt to epi_slide, epi_slide_fast, etc
• Making epi_slide_opt accept basic functions like mean and median and dispatching to our favored specialized sliding function implementation like frollmean on detecting mean; otherwise frollapply-ing the function
• Removing current epi_slide
• Making epi_slide perform dplyr-like magic to dispatch to use fast rolling functions where possible
• etc.; see Slack thread here

[brookslogan]

Related: #629