We had an ordered set type (set.Slice) already but we occasionally want
to do the same thing with a map, preserving the order things were added,
so add that too, as mapsx.OrderedMap[K, V], and then use in ipnext.
Updates #12614
Change-Id: I85e6f5e11035571a28316441075e952aef9a0863
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>