Simplify struct creations with the `Default` trait

[tomassedovic]

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For example, the Object::new method could be simplified to something like this:

Object {
    x: x,
    y: y,
    char: char,
    color: color,
    name: name.into(),
    blocks: blocks,
    level: 1,
    .. default::Default(),
}

Removing all the None and false values. That would simplify the tutorial as we wouldn't have to change the structs so often. The Game one seems like another likely candidate.