Arms now have Content, add Furnaces

Instead of just connecting two Structures, Arms now hold Items themselves and behave more similar to Belts.
I also added the new Structure Furnace. It can convert Ores to Plates while using Coal as fuel.
With those changes the Material type now looks like this:

pub enum Material {
    Ore(Ore),
    Plate(Plate),
    Wood,
    Coal,
}

pub enum Ore {
    Iron,
    Copper,
}

pub enum Plate {
    Iron,
    Copper,
}

To support the conversion from Ore to Plate within the Furnace I added a function to Ore:

impl Ore {
    pub fn smelted(&self) -> Plate {
        match self {
            Self::Iron => Plate::Iron,
            Self::Copper => Plate::Copper,
        }
    }
}

The basic implementation of the Furnace then looks like this:

pub struct Furnace {
    pub coal: Option<Material>,
    pub ore: Option<Ore>,
    pub product: Option<Plate>,
}

impl Furnace {
    pub fn step(&mut self) -> bool {
        match (self.product, &self.coal, self.ore) {
            (None, Some(Material::Coal), Some(ore)) => {
                self.coal = None;
                self.ore = None;
                self.product = Some(ore.smelted());
                true
            }
            _ => false,
        }
    }
}

With step returning whether anything changed.

Since there’s now a Furnace I updated the example map to:

<<<<<<<   
      a   
c>>>>eFE<<
         ^
))))>v   ^
     v   ^
     U   a
     >>)>>