This leaves behind a type alias and associated constructor, to allow
for gradual switchover.
Updates #3206.
Signed-off-by: David Anderson <danderson@tailscale.com>
Complete with converters to all the other types that represent a
node key today, so the new type can gradually subsume old ones.
Updates #3206
Signed-off-by: David Anderson <danderson@tailscale.com>