Support defaultable types for adding table rows

[molpopgen]

The following pseudo code shows how struct update syntax can be used to provide API sugar w/o default arguments:

#[derive(Copy, Clone, Debug, Hash)]
struct NodeDefaults {
    flags: i32,
    population: i32,
    individual: i32,
}

impl Default for NodeDefaults {
    fn default() -> Self {
        Self {
            flags: 0,
            population: -1,
            individual: -1,
        }
    }
}

fn add_node_with_defaults(_time: f64, _defaults: NodeDefaults) {}

fn main() {
    add_node_with_defaults(0.0, NodeDefaults::default());

    let defaults = NodeDefaults {
        population: 2,
        ..Default::default()
    };
    assert_eq!(defaults.population, 2);
    assert_eq!(defaults.individual, -1);
    add_node_with_defaults(0.0, defaults);
    // Type is Copy, so we can re-use
    add_node_with_defaults(0.0, defaults);

    let defaults2 = NodeDefaults {
        individual: 2,
        ..defaults
    };
    assert_eq!(defaults2.population, 2);
    assert_eq!(defaults2.individual, 2);
    add_node_with_defaults(0.0, defaults2);
}

See this blog post for discussion.

[molpopgen]

#466 implemented this for node table rows.

[molpopgen]

closed by #448