SimpleStructure

Up until now the Item type looked like this:

 pub enum Item {
     Material(Material),
     Structure(Structure),
     ...
 }

While Structure is either one of the concrete buildings:

pub enum Structure {
    Belt(Belt),
    Arm(Arm),
    LongArm(LongArm),
    Source(Source),
    Target(Target),
    ...
}

Since those types also track values such as their cooldown and content it becomes very wasteful and complicated to work with Item. A Belt could transport another Belt which could have another Structure as content itself.
This is why until now most Structures were basically unable to hold Item and only worked with Material. Also some of the additional information is irrelevant when e.g. an Item is transported on a Belt.
I therefore introduced SimpleStructure which is just a traditional enum without any additional data per variant.

pub enum SimpleStructure {
    Belt,
    Arm,
    LongArm,
    Source,
    Target,
    ...
}

Item now updated to use SimpleStructure instead of Structure:

pub enum Item {
    Material(Material),
    Structure(SimpleStructure),
    ...
}

For easier usage I also added conversion methods such as:

impl From<SimpleStructure> for Structure {
    ...
}

impl From<&Structure> for SimpleStructure {
    ...
}