Add `.upsert()` impl

[trevyn]

With SQLite INSERT OR REPLACE INTO

or

execute!(
    "INSERT INTO node(host, id)"
    "VALUES (" host, node.id ")"
    "ON CONFLICT(host) DO UPDATE SET id = " node.id
)?;

Node {
    host: Some(host),
    id: Some(node.id),
    ..Default::default()
}.upsert_on("host")?;

upsert!(Node { host, id: node.id });

insert!(Node { host, id: node.id });

Oh dear.