Represent positions with i32 rather than usize

Even though we can now represent the three empty negative quadrants,
it'll make the math simpler to implement so I figure there's no reason
not to do it.

src/components.rs

@@ -6,8 +6,8 @@ use specs_derive::Component;
 /// Entities that have a physical position in the world.
 #[derive(Component)]
 pub struct Position {
-    pub x: usize,
-    pub y: usize,
+    pub x: i32,
+    pub y: i32,
 }
 
 /// Entities that need to be drawn as a single character.

src/systems.rs

@@ -37,8 +37,8 @@ impl<'a> System<'a> for MobSystem {
             match mob.next_action {
                 MobAction::Nop => {}
                 MobAction::Move(dx, dy) => {
-                    pos.x = (pos.x as i32 + dx) as _;
-                    pos.y = (pos.y as i32 + dy) as _;
+                    pos.x = pos.x + dx;
+                    pos.y = pos.y + dy;
                 }
             }