Add physics

src/main.rs

 mod components;
+mod physics;
 
 use sdl2::pixels::Color;
 use sdl2::event::Event;

src/physics.rs

+use specs::prelude::*;
+
+use crate::components::*;
+
+struct Physics;
+
+impl<'a> System<'a> for Physics {
+    type SystemData = (WriteStorage<'a, Position>, ReadStorage<'a, Velocity>);
+
+    fn run(&mut self, mut data: Self::SystemData) {
+        use self::Direction::*;
+
+        for (pos, vel) in (&mut data.0, &data.1).join() {
+            match vel.direction {
+                Left => {
+                    pos.0 = pos.0.offset(-vel.speed, 0);
+                },
+                Right => {
+                    pos.0 = pos.0.offset(vel.speed, 0);
+                },
+                Up => {
+                    pos.0 = pos.0.offset(0, -vel.speed);
+                },
+                Down => {
+                    pos.0 = pos.0.offset(0, vel.speed);
+                },
+            }
+        }
+    }
+}