Add impl to Note that finds outgoing links

[Knut Magnus Aasrud]

This is already done during serialization:

links: note
    .adjacencies
    .iter()
    .filter(|(_, edge)| matches!(edge, Edge::Connected(_)))
    .map(|(id, _)| hex(id))
    .collect::<Vec<String>>()

This should simply be added as a utility function on Note.