util/set: make Set.Slice return elements in sorted order for ordered types

This makes Set.MarshalJSON produce deterministic output in many cases now.
We still need to do make it deterministic for non-ordered types.

Updates #18808

Change-Id: I7f341ec039c661a8e88d07d7f4dc0f15d5d4ab86
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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