Replace inherent setter methods with a `Set` trait

[japaric]

The current setter methods are verbose now that enums are namespaced:

curve(.., |c| c.
    color(Color::DarkViolet).
    label("sin(x)").
    line_type(LineType::Dash).
    point_type(PointType::Circle)).

These methods could be replaced with the following trait:

trait Set<T> {
    fn set(&mut self, T) -> &mut Self;
}

Now the chain would look like this:

curve(/* .. */, |c| c.
    set(Color::DarkViolet).
    set(Label("sin(x)")).
    set(LineType::Dash).
    set(PointType::Circle)).