For better readability.

Make it clear that you need to restart in order for icons to refresh
properly.

The Agua system icons I'm using don't include all of the variants
required on Mac OS X 10.12 Sierra. The originals have seven sizes (32,
256, 512, and 1024), while the system icons have 10 (16, 32, 64, 128,
256, 512, 1024).

Using the seven-size icns didn't work when the file was copied into
`CoreTypes.bundle`. When I added additional icns size variants, the
replacement did work.

This script abstracts the manual work I did to add the appropriate
variants to the Agua icons. Other icon sets may not convert in the same
way.

Dock.app needs to be changed to include the Finder and both states of
the Trash icon.

Learned how to convert .icns files to PNG from:
https://www.junian.net/tech/icns-to-png/

* It turns out not all icons I was trying to replace live in
  `CoreTypes.bundle`. They're instead spread out over various Kexts and
  bundles. Update the script to look in all of these places (except for
  the Dock, haven't done that yet because the .icns files need to be
  converted to PNGs in order to set up the replacement) and replace
  icons in their respective locations.
* Now that we're modifying more bundles, we need to back up all of them.
  Replace the `cp`-based backup with a "tar gz"-based one. Use a
  timestamp on the backup file names to ensure we don't accidentally
  overwrite an icon/bundle backup.

Copies all icon files into `CoreTypes.bundle` and resets the icon cache.
The computer will need to be restarted, or the user account
re-logged-into in order for the changes to take effect.

Leveraged the following scripts for the copy semantics and icon cache
clearing instructions:
- https://github.com/DocSystem/bigsur-icons-for-catalina/blob/14b8c406db69f6c48639e8dfd91b4c4e1cc9bf84/applyIcons.command
- https://github.com/w0lfschild/ClassicFolders/blob/9221de592915aee3eb11dc471db95645e97bc566/classic_folders.command