Marshal as a JSON list instead of a map. Because set elements are
`comparable` and not `cmp.Ordered`, we cannot easily sort the items
before marshaling.
Updates #cleanup
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
We have two other types of Sets here. Add the basic obvious one too.
Needed for a change elsewhere.
Updates #cleanup
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>