Substitute for optional doesn't work right

[AldaronLau]

With this MuON file:

sig:
sig: Allegro
  time_symbol: true
  swing_text: Swing = 60%

And this Rust code:

#[derive(PartialEq, Debug, Serialize, Deserialize)]
pub struct SigStyle {
    pub tempo: Option<String>,
    pub time_symbol: bool,
    pub swing_text: Option<String>,
}

#[derive(PartialEq, Debug, Default, Serialize, Deserialize)]
pub struct Style {
    pub sig: Vec<SigStyle>,
}

fn main() {
    let input = include_str!("../scof/Style.muon");

    let style: Style = muon::from_str(input).unwrap();

    println!("{:?}", style);

    let output = muon::to_string(&style).unwrap();

    println!("{}", output);

    let style_clone: Style = muon::from_str(&output).unwrap();

    assert_eq!(style, style_clone);
    assert_eq!(input, output);
}

This panics with a FailedParse("bool: Allegro")

[DougLau]

Substitution not supported with optional types, but the error should make sense with the latest commits.