Crossing Structure

I added the new Structure Crossing. It allows the movement of Items both vertically and horizontally.
The direction from where the last Item was placed onto it then determines its horizontal or vertical movement direction.

pub struct Crossing {
    left: Option<Material>,
    right: Option<Material>,
    top: Option<Material>,
    bottom: Option<Material>,
    horizontally: Horizontally,
    vertically: Vertically,
    ...
}

pub enum Horizontally {
    Right,
    Left,
}

pub enum Vertically {
    Up,
    Down,
}

The Crossing needs to know from which direction Items it accepts are coming from, to decide on one of the four possible slots and movement directions.
I therefore had to extend some of the ItemHandler‘s functions from:

pub trait ItemHandler {
    fn accepts(
        &self,
        pos: &PosRelative,
        x: &Item,
        ...
    ) -> bool;

    fn accept(
        &mut self,
        pos: &PosRelative,
        x: Item,
        ...
    );
    ...

to

pub trait ItemHandler {
    fn accepts(
        &self,
        pos_self: &PosRelative,
        pos_other: &PosRelative,
        x: &Item,
        ...
    ) -> bool;

    fn accept(
        &mut self,
        pos_self: &PosRelative,
        pos_other: &PosRelative,
        x: Item,
        ...
    );
    ...